RFA: AVR: add infrastructure for device packages

As the steering commitee still hasn't spoken on the maintainership issue,
apparently this still has to go the write-after-approval route.

The purpose of this patch is to make it possible to add support for new
devices (MCUs) to the AVR toolchain, without having to re-build the
entire toolchain.  This capability is desirable because new MCUs are added
fairly frequently.

There are multiple parts of the toolchain involved.
gcc changes multilibbing to key off the new -march option; the -mmcu option
is translated via DRIVER_SELF_SPECS into a -specs option, and the
individual spec files contain the required settings like -march, and various
more detailed settings (some of which are for new options).

binutils provides new relocation and relaxation facilities to allow referring
symbolically to symbol differences and/or I/O addresses.
avr-libc puts the device-specifc header settings in avr/io*.h, and a few
small device-specific likbale functions into a device-specific library.

The other toolchain parts are staged here:
git@github.com:embecosm/avr-binutils-gdb.git avr-mainline
git@github.com:embecosm/avr-libc.git avr-libc-embecosm-mainline

Attached is the GCC patch for the basic device package infrastructure.
OK to apply?

I intend to send the patch sets for avrtiny support (modified for device
package support) and for libstdc++-v3 support next, each building on the
previous patch set.  The avrtiny support is also dependent on the
caller-save patch https://gcc.gnu.org/ml/gcc-patches/2014-10/msg00420.html ,
as avr-libc won't build otherwise.
2014-10-08  Joern Rennecke  <joern.rennecke@embecosm.com>

	* config/avr/avr.opt (mmcu=): Change to have a string value.
	(mn-flash=, mskip-bug, march=, mrmw): New options.
	(HeaderInclude): New.
	(mmcu=): Remove Var / Init clauses.
	* config/avr/avr.h (DRIVER_SELF_SPECS): Translate -mmcu into a
	-specs option.
	(SYMBOL_FLAG_IO, SYMBOL_FLAG_ADDRESS): Define.
	(ASM_OUTPUT_ALIGNED_BSS): Use avr_asm_asm_output_aligned_bss.
	(SYMBOL_FLAG_IO_LOW): Define.
	(avr_device_to_as, avr_device_to_ld): Don't declare.
	(avr_device_to_data_start, avr_device_to_startfiles): Likewise.
	(avr_device_to_devicelib, avr_device_to_sp8): Likewise.
	(EXTRA_SPEC_FUNCTIONS): Don't define.
	(ASM_SPEC): Translate -arch= option to -mmcu= option.
	(LINK_SPEC): Translate -arch= option to -m= option.
	Don't use device_to_ld / device_to_data_start.
	(STARTFILE_SPEC): Now empty.
	(ASM_SPEC): Add -%{mrelax: --mlink-relax}.
	* config/avr/gen-avr-mmcu-specs.c: New file.
	* config/avr/t-avr (gen-avr-mmcu-specs$(build_exeext)): New rule.
	(s-device-specs): Likewise.
	(GCC_PASSES): Add s-device-specs.
	(install-driver): Depend on install-device-specs.
	(install-device-specs): New rule.
	* config/avr/avr.c (avr_option_override): Look up mcu arch by
	avr_arch_index and provide fallback initialization for avr_n_flash.
	(varasm.h): #include.
	(avr_print_operand) <i>: Allow SYMBOL_REF with SYMBOL_FLAG_IO;
	(avr_handle_addr_attribute, avr_eval_addr_attrib): New functions.
	(avr_attribute_table): Add "io", "address" and "io_low".
	(avr_asm_output_aligned_decl_common): Change type of decl to tree.
	Add special handling for symbols with "io" and/or "address" attributes.
	(avr_asm_asm_output_aligned_bss): New function.
	(avr_encode_section_info): Set SYMBOL_FLAG_IO and SYMBOL_FLAG_ADDRESS
	as appropriate.  Handle io_low attribute.
	(avr_out_sbxx_branch): Handle symbolic io addresses.
	(avr_xload_libgcc_p, avr_nonconst_pointer_addrspace): Use
	avr_n_flash instead of avr_current_device->n_flash.
	(avr_pgm_check_var_decl, avr_insert_attributes): Likewise.
	(avr_emit_movmemhi): Likewise.
	* config/avr/avr-c.c (avr_cpu_cpp_builtins): Likewise.
	Use TARGET_RMW instead of avr_current_device->dev_attributes.
	Don't define avr_current_device->macro (that's the specfile's job).
	Use TARGET_SKIP_BUG instead of avr_current_device->errata_skip.
	* config/avr/avr.c (avr_2word_insn_p): Likewise.
	* config/avr/avr.md (*cpse.ne): Likewise.
	(mov<mode>): Use avr_eval_addr_attrib.
	(cbi): Change constraint for low_io_address_operand operand to "i".
	(sbi, sbix_branch, sbix_branch_bit7, insv.io, insv.not.io): Likewise.
	* config/avr/predicates.md (io_address_operand):
	Allow SYMBOL_REF with SYMBOL_FLAG_IO.
	(low_io_address_operand): Allow SYMBOL_REF with SYMBOL_FLAG_IO_LOW.
	* config/avr/avr-protos.h (avr_asm_output_aligned_decl_common):
	Update prototype.
	(avr_eval_addr_attrib, avr_asm_asm_output_aligned_bss): Prototype.
	* config/avr/genmultilib.awk: Use -march=.
	Remove Multilib matches processing.
	* config/avr/t-multilib, config/avr/avr-tables.opt: Regenerate.
	* config/avr/avr-arch.h: Add double include guard.
	(avr_mcu_t) <library_name>: Update comment.
	* config/avr/driver-avr.c (avr_device_to_as): Delete.
	(avr_device_to_ld, avr_device_to_data_start): Likewise.
	(avr_device_to_startfiles, avr_device_to_devicelib): Likewise.
	(avr_device_to_sp8): Likewise.
	* config/avr/genopt.sh:  Instead avr_mcu, emit an Enum for avr_arch.

	* doc/extend.texi (io, address): Document new AVR variable attributes.
	(io_low): Likewise.

RFA: AVR: add infrastructure for device packages

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