Handle RDRAND failures.

I mistakenly believed that only RDSEED could fail. However, the Intel
manuals state that RDRAND can fail too.

This change cherry-picks the following BoringSSL changes:

2cac3506 – Handle RDRAND failures.
248abbd7 – Add missing comma in .type pragma for rdrand code.

Change-Id: Icdc56a50ce36e9c525063583882c676a5312d313
7 files changed