Merge remote-tracking branch 'remotes/rth/tags/pull-tcg-20171103' into staging

Queued tcg patches

# gpg: Signature made Fri 03 Nov 2017 08:37:58 GMT
# gpg:                using RSA key 0x64DF38E8AF7E215F
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>"
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-tcg-20171103:
  cpu-exec: Exit exclusive region on longjmp from step_atomic
  tcg/s390x: Use constant pool for prologue
  tcg: Allow constant pool entries in the prologue

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>