The former is defined by the kernel and is a straightforward number. The
latter is defined by glibc as:

  #define SYS_getrandom __NR_getrandom

which does not work when kernel headers are older than glibc headers.
Instead, use the kernel values.

Bug: chromium:742260
Change-Id: Id162f125db660643269e0b1329633437048575c4
Reviewed-on: https://boringssl-review.googlesource.com/17864
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>