MC: handle .cfi_startproc simple

The statement

  .cfi_startproc simple

is perfectly valid, and GNU as supports it.  Unfortunately, LLVM barfs
with:

  error: invalid instruction mnemonic 'simple'

Fix this by modifying ParseDirectiveCFIStartProc() to parse the
identifier "simple".

Cc: Rafael Espindola <rafael.espindola@gmail.com>
Cc: Jim Grosbach <grosbach@apple.com>
Cc: Eli Friedman <eli.friedman@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>