Merge remote-tracking branch 'remotes/rth/tags/pull-tcg-20210323' into staging

Workaround for macos mprotect
Workaround for target_page vs -flto

# gpg: Signature made Wed 24 Mar 2021 01:40:12 GMT
# gpg:                using RSA key 7A481E78868B4DB6A85A05C064DF38E8AF7E215F
# gpg:                issuer "richard.henderson@linaro.org"
# 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-20210323:
  exec: Build page-vary-common.c with -fno-lto
  exec: Extract 'page-vary.h' header
  exec: Rename exec-vary.c as page-vary.c
  tcg: Workaround macOS 11.2 mprotect bug
  tcg: Do not set guard pages on the rx portion of code_gen_buffer

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