x86emul: AVX512-FP16 testing

Naming of some of the builtins isn't fully consistent with that of pre-
existing ones, so there's a need for a new BR2() wrapper macro.

With the tests providing some proof of proper functioning of the
emulator code also enable use of the feature by guests, as there's no
other infrastructure involved in enabling this ISA extension.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Henry Wang <Henry.Wang@arm.com> # CHANGELOG

CHANGELOG.md

@@ -20,6 +20,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - Bus-lock detection, used by Xen to mitigate (by rate-limiting) the system
      wide impact of a guest misusing atomic instructions.
  - xl/libxl can customize SMBIOS strings for HVM guests.
+ - Add support for AVX512-FP16 on x86.
 
 ## [4.17.0](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.17.0) - 2022-12-12