Align pointers by hand.

This avoids having Windows be different and is also easier for testing
because it's a simple matter to unalign the pointer if needed.

Change-Id: I32cfa5834e3fe4f16304a25092b9c71946d4744d
Reviewed-on: https://boringssl-review.googlesource.com/3131
Reviewed-by: Adam Langley <agl@google.com>