Merge remote-tracking branch 'remotes/rth-gitlab/tags/pull-hex-20210306' into staging

Add hexagon to include/exec/poison.h
Two Coverity fixes for target/hexagon/

# gpg: Signature made Sun 07 Mar 2021 01:37:05 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-gitlab/tags/pull-hex-20210306:
  target/hexagon/opcodes: Add missing varargs cleanup
  target/hexagon: Fix shift amount check in fASHIFTL/fLSHIFTR
  exec: Poison Hexagon target-specific definitions

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