As some of ARM processors, more specifically Cortex-Mx series, are
Thumb2-only, we need to support Thumb2-only builds even in assembly.
(Imported from upstream's 11208dcfb9105e8afa37233185decefd45e89e17.)
Change-Id: I7cb48ce6a842cf3cfdf553f6e6e6227d52d525c0
Reviewed-on: https://boringssl-review.googlesource.com/17108
Reviewed-by: Adam Langley <agl@google.com>