MSAN doesn't hook |syscall| and thus doesn't know that the kernel has
filled the output buffer when |getrandom| is called.

This change tells MSAN to trust that the memory that |getrandom| writes
to has been initialised. This should avoid false-positives when code
operates on |RAND_bytes| output.

Change-Id: I0a74ebb21bcd1de1f28eda69558ee27f82db807a
Reviewed-on: https://boringssl-review.googlesource.com/12903
Reviewed-by: Adam Langley <alangley@gmail.com>
Commit-Queue: Adam Langley <alangley@gmail.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>