X86: disambiguate unqualified btr, bts

The instruction mnemonics btr and bts are perfectly valid, and have
existed since Intel 386.  GNU as supports them fine, and there are
instances of its usage in linux.git.  Unfortunately, LLVM does not
support them, and barfs with:

  error: ambiguous instructions require an explicit suffix

Fix this problem by disambiguating it just like GNU as does, following
the example set by 824a907.

Cc: Eli Friedman <eli.friedman@gmail.com>
Cc: Jim Grosbach <grosbach@apple.com>
Cc: PaX Team <pageexec@gmail.com>
Cc: Chris Lattner <sabre@nondot.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>