[keymaster] fix -Wimplicit-fallthrough

Clang now requires an explicit attribute for C code.

Add feature detection for the GNU C attribute, and annotate the source
using it.

Bug: 139945549
Test: mm
Change-Id: I9c047f0c607632ade8b326ac3a793b3879ba4a5d
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
1 file changed