ARM assembly pack: get ARMv7 instruction endianness right.

Pointer out and suggested by: Ard Biesheuvel.

(Imported from upstream's 5dcf70a1c57c2019bfad640fe14fd4a73212860a)

This is from a while ago, but it's one source of divergence between our copy of
these files and master's.

Change-Id: I6525a27f25eb86a92420c32996af47ecc42ee020
Reviewed-on: https://boringssl-review.googlesource.com/4487
Reviewed-by: Adam Langley <agl@google.com>