bn_mul.h: fix x86 PIC ASM compilation with GCC<5

Fixes Mbed-TLS#1910

With ebx added to the MULADDC_STOP clobber list to fix Mbed-TLS#1550, the inline
assembly fails to build with GCC<5 in PIC mode with the following error:

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, detect this situation and disable the inline assembly,
similar to the MULADDC_CANNOT_USE_R7 logic.

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

include/mbedtls/bn_mul.h

@@ -50,13 +50,29 @@
 #if defined(__GNUC__) && \
     ( !defined(__ARMCC_VERSION) || __ARMCC_VERSION >= 6000000 )
 
+/*
+ * GCC < 5.0 treated the x86 ebx (which is used for the GOT) as a
+ * fixed reserved register when building as PIC, leading to errors
+ * like: bn_mul.h:46:13: error: PIC register clobbered by ‘ebx’ in ‘asm’
+ *
+ * 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.
+ */
+#if defined(__GNUC__) && __GNUC__ < 5 && defined(__PIC__)
+#define MULADDC_CANNOT_USE_EBX
+#endif
+
 /*
  * Disable use of the i386 assembly code below if option -O0, to disable all
  * compiler optimisations, is passed, detected with __OPTIMIZE__
  * This is done as the number of registers used in the assembly code doesn't
  * work with the -O0 option.
  */
-#if defined(__i386__) && defined(__OPTIMIZE__)
+#if defined(__i386__) && defined(__OPTIMIZE__) && !defined(MULADDC_CANNOT_USE_EBX)
 
 #define MULADDC_INIT                        \
     asm(                                    \