This change causes the generated assembly files for ARM and AArch64 to
have #if guards for __arm__ and __aarch64__, respectively. Since
building on ARM is only supported for Linux, we only have to worry about
GCC/Clang's predefines.

Change-Id: I7198eab6230bcfc26257f0fb6a0cc3166df0bb29
Reviewed-on: https://boringssl-review.googlesource.com/5173
Reviewed-by: Adam Langley <agl@google.com>