(There are several other defines in sysdeps/arm/nacl/arm-features.h that
are never defined for non-NaCl, or have constant definitions for non-NaCl
- ARM_ALWAYS_BX, ARM_BX_ALIGN_LOG2, ARM_NO_INDEX_REGISTER - and which
should probably be cleaned up as well eventually in separate patches, but
the sfi_* are by far the most invasive. ARM_ASSUME_NO_IWMMXT is only
defined for NaCl at present, but logically could be defined for all builds
for ARMv6 and later (__ARM_ARCH >= 6, given GCC 4.8 or later or the
__ARM_ARCH emulation in sysdep.h) as iWMMXt is only on certain v5TE
processors. NO_THUMB is more complicated in that only NaCl defines it in
arm-features.h, but some individual .S files define it as well.)