Merge remote-tracking branch 'remotes/rth/tags/pull-tcg-20190221' into staging

Allow const void * as argument to helpers.
Remove obsolete TODO file.

# gpg: Signature made Thu 21 Feb 2019 18:59:11 GMT
# gpg:                using RSA key 64DF38E8AF7E215F
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>" [full]
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-tcg-20190221:
  include/exec/helper-head.h: support "const void *" in helper calls
  tcg: Remove TODO file

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