Add (void) to some macros to satisfy compiler.

More modern versions of GCC (at least with aarch64) are warning about an
unused value in these locations. It's incorrect, but I guess that the
macro is confusing it.

Using a (void) tag is a little ugly but solves the problem.

Change-Id: If6ba5083ab6e501c81e7743ae1ed99a89565e57c
Reviewed-on: https://boringssl-review.googlesource.com/2810
Reviewed-by: Adam Langley <agl@google.com>