[compiler-rt] Add bitmask to fix warning (#187812)

After #186881 was merged the gcc libc bots started complaining about the
conversion from u8 to 2 bit integer being unsafe (see:
https://lab.llvm.org/buildbot/#/builders/131/builds/42788). This PR
adds a bitmask that fixes the warning.

GitOrigin-RevId: 6891a6ef0e2466ffb8a921d7f58934ef464f137a
1 file changed