tests/cpu-policy: Skip building on older versions of GCC

tests/cpu-policy: Skip building on older versions of GCC

GCC 4.4 (as included in CentOS 6) is too old to handle designated initialisers
in anonymous unions.  As this is just a developer tool, skip the test in this
case, rather than sacraficing the legibility/expresibility of the test cases.

This fixes the Gitlab CI tests.

While adding this logic to cpu-polcy, adjust the equivelent logic from
x86_emulator on which this was based.  Printing:

  Test harness not built, use newer compiler than "gcc"

isn't helpful for anyone unexpectedly encountering the error.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>