security_x86Registers: add BIOS BILD bit check

This adds a check for the BILD bit in the RCBA GCS register, along with
infrastructure to support it.

BUG=chromium-os:23906
TEST=link build, security_x86Registers passes

Change-Id: Ie84b28e681f709422c85b51d486f72cf9e1199a1
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/42593
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
2 files changed