ARMv8 kindly deprecated most of its IT instructions in Thumb mode.
These files are taken from upstream and are used on both ARMv7 and ARMv8
processors. Accordingly, silence the warnings by marking the file as
targetting ARMv7. In other files, they were accidentally silenced anyway
by way of the existing .arch lines.

This can be reproduced by building with the new NDK and passing
-DCMAKE_ASM_FLAGS=-march=armv8-a. Some of our downstream code ends up
passing that to the assembly.

Note this change does not attempt to arrange for ARMv8-A/T32 to get
code which honors the constraints. It only silences the warnings and
continues to give it the same ARMv7-A/Thumb-2 code that backwards
compatibility dictates it continue to run.

Bug: chromium:575886, b/63131949
Change-Id: I24ce0b695942eaac799347922b243353b43ad7df
Reviewed-on: https://boringssl-review.googlesource.com/24166
Reviewed-by: Adam Langley <agl@google.com>