Surround immintrin.h includes with warning pragmas.

smmintrin.h uses #if __cplusplus rather than #ifdef __cplusplus which
trips up warnings.

Change-Id: Ic2e0a64f0485a6e1d807c1e2d9d7f5bf8c64e296
Reviewed-on: https://boringssl-review.googlesource.com/5450
Reviewed-by: Adam Langley <agl@google.com>