clang: disable the gcc-compat warnings for read_atomic

clang: disable the gcc-compat warnings for read_atomic

clang gcc-compat warnings can wrongly fire when certain constructions are used,
at least the following flow:

switch ( ... )
{
case ...:
    while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
    {
        ...

Will cause clang to emit the following warning "'break' is bound to loop, GCC
binds it to switch", which is a false positive, and both gcc and clang bind
the break to the inner switch. In order to workaround this issue, disable the
gcc-compat checks for the usage of the read_atomic macro.

This has been reported upstream as http://bugs.llvm.org/show_bug.cgi?id=32595.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>