mbedtls: fix x86 PIC build with GCC < 5

Fixes:
http://autobuild.buildroot.net/results/d6d/d6dc9a640aa1f6650a3e7b9397f2fe2ae3433f4d/
http://autobuild.buildroot.net/results/ab5/ab5a58ea7845f9f378454ee1aa7e872448618ba9/

ebx was recently added to the x86 inline asm MULADDC_STOP clobber list to
fix #1550, but this causes the build to fail with GCC < 5 when building in
PIC mode with errors like:

include/mbedtls/bn_mul.h:46:13: error: PIC register clobbered by ‘ebx’ in ‘asm’

This is because older GCC versions treated the x86 ebx register (which is
used for the GOT) as a fixed reserved register when building as PIC.

This is fixed by an improved register allocator in GCC 5+.  From the release
notes:

Register allocation improvements: Reuse of the PIC hard register, instead of
using a fixed register, was implemented on x86/x86-64 targets.  This
improves generated PIC code performance as more hard registers can be used.

https://www.gnu.org/software/gcc/gcc-5/changes.html

As a workaround, add a patch to detect this situation and disable the inline
assembly, similar to the MULADDC_CANNOT_USE_R7 logic.

Patch submitted upstream: https://github.com/ARMmbed/mbedtls/pull/1986



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>