But for the ABI conversion bits, these are just leaf functions and don't
even need unwind tables. Just renumber the registers on Windows to only
used volatile ones.

In doing so, this switches to writing rdrand explicitly. perlasm already
knows how to manually encode it and our minimum assembler versions
surely cover rdrand by now anyway. Also add the .size directive. I'm not
sure what it's used for, but the other files have it.

(This isn't a generally reusable technique. The more complex functions
will need actual unwind codes.)

Bug: 259
Change-Id: I1d5669bcf8b6e34939885d78aea6f60597be1528
Reviewed-on: https://boringssl-review.googlesource.com/c/34867
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>