Merge remote-tracking branch 'remotes/rth/tags/pull-tcg-20170117' into staging

tcg/i386 fixes

# gpg: Signature made Tue 17 Jan 2017 22:58:04 GMT
# gpg:                using RSA key 0xAD1270CC4DD0279B
# gpg: Good signature from "Richard Henderson <[email protected]>"
# gpg:                 aka "Richard Henderson <[email protected]>"
# gpg:                 aka "Richard Henderson <[email protected]>"
# Primary key fingerprint: 9CB1 8DDA F8E8 49AD 2AFC  16A4 AD12 70CC 4DD0 279B

* remotes/rth/tags/pull-tcg-20170117:
  tcg/i386: Always use TZCNT when available
  Revert "tcg/i386: Rely on undefined/undocumented behaviour of BSF/BSR"

Signed-off-by: Peter Maydell <[email protected]>