Merge remote-tracking branch 'remotes/rth-gitlab/tags/pull-c11-20210615' into staging

Change to -std=gnu11.
Replace QEMU_GENERIC with _Generic.
Remove configure detect of _Static_assert.

# gpg: Signature made Wed 16 Jun 2021 02:32:32 BST
# gpg:                using RSA key 7A481E78868B4DB6A85A05C064DF38E8AF7E215F
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* remotes/rth-gitlab/tags/pull-c11-20210615:
  configure: Remove probe for _Static_assert
  qemu/compiler: Remove QEMU_GENERIC
  include/qemu/lockable: Use _Generic instead of QEMU_GENERIC
  util: Use unique type for QemuRecMutex in thread-posix.h
  util: Pass file+line to qemu_rec_mutex_unlock_impl
  util: Use real functions for thread-posix QemuRecMutex
  softfloat: Use _Generic instead of QEMU_GENERIC
  configure: Use -std=gnu11

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