Add consts to casts to suppress warnings.

Pull the "cast this to a pointer to an __m128i" idiom into a macro, and
use the macro; have that macro use "const" in the casts.

Change-Id: Ife90b7daef2f09368790f3b2ffbb227d6d794dea
Reviewed-on: https://code.wireshark.org/review/2871
Reviewed-by: Guy Harris <guy@alum.mit.edu>