libnspr: only enable thumb if BR2_ARM_INSTRUCTIONS_THUMB2

libnspr currently passes --enable-thumb2 if the CPU has thumb
instructions. This option will pass -mthumb to the compiler. However,
if an external multilib toolchain is used that has a thumb-specific
variant (e.g. Sourcery), it will try to use that one. But we only copy
a single variant to the sysroot, so the build will fail with:

.../arm-none-linux-gnueabi/bin/ld: cannot find crti.o: No such file or directory
...
collect2: error: ld returned 1 exit status
../../config/rules.mk:303: recipe for target 'libnspr4.so' failed

In order to fix this, only enable thumb2 when we are really using
thumb2. Note that it is still necessary to pass this option, cfr.
http://autobuild.buildroot.org/results/d7323831372050e425a34f5104a46d8cbd6be214

We could have chosen to still enable thumb2 with the internal toolchain.
However, that would mean we create a discrepancy between the result of
building with the same (buildroot) toolchain when it is used as an
internal one or as an external one. To avoid potentially surprising
results, just stick to the option chosen for the rest of the rootfs.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
---
Build-tested with various combinations of thumb, thumb2, floating point,
and external toolchains. I didn't test with an internal toolchain but
what could go wrong there? :-)

Changes v2: added missing build failure output to commit message
---
 package/libnspr/libnspr.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

>>>>> "Arnout" == Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be> writes:

 > libnspr currently passes --enable-thumb2 if the CPU has thumb
 > instructions. This option will pass -mthumb to the compiler. However,
 > if an external multilib toolchain is used that has a thumb-specific
 > variant (e.g. Sourcery), it will try to use that one. But we only copy
 > a single variant to the sysroot, so the build will fail with:

 > .../arm-none-linux-gnueabi/bin/ld: cannot find crti.o: No such file or directory
 > ...
 > collect2: error: ld returned 1 exit status
 > ../../config/rules.mk:303: recipe for target 'libnspr4.so' failed

 > In order to fix this, only enable thumb2 when we are really using
 > thumb2. Note that it is still necessary to pass this option, cfr.
 > http://autobuild.buildroot.org/results/d7323831372050e425a34f5104a46d8cbd6be214

 > We could have chosen to still enable thumb2 with the internal toolchain.
 > However, that would mean we create a discrepancy between the result of
 > building with the same (buildroot) toolchain when it is used as an
 > internal one or as an external one. To avoid potentially surprising
 > results, just stick to the option chosen for the rest of the rootfs.

 > Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
 > ---
 > Build-tested with various combinations of thumb, thumb2, floating point,
 > and external toolchains. I didn't test with an internal toolchain but
 > what could go wrong there? :-)

 > Changes v2: added missing build failure output to commit message
 > ---
 >  package/libnspr/libnspr.mk | 2 +-
 >  1 file changed, 1 insertion(+), 1 deletion(-)

 > diff --git a/package/libnspr/libnspr.mk b/package/libnspr/libnspr.mk
 > index 8e58986..c9bfa0c 100644
 > --- a/package/libnspr/libnspr.mk
 > +++ b/package/libnspr/libnspr.mk
 > @@ -50,7 +50,7 @@ LIBNSPR_INSTALL_STAGING_OPTS = DESTDIR=$(STAGING_DIR) LIBRARY= install
 >  endif
 
 >  ifeq ($(BR2_arm),y)
 > -ifeq ($(BR2_ARM_CPU_HAS_THUMB2),y)
 > +ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB2),y)
 >  LIBNSPR_CONF_OPTS += --enable-thumb2

Thanks. The conclusion was that --enable-thumb2 just caused it to pass
-mthumb in CFLAGS (and --disable-thumb2 to pass -marm), right?

What should we do for classic ARM cores in thumb1 mode? There we afaik
also want to use --enable-thumb2, but it could potentially break in
future if the option is changed to do something more than this.