x86: rename HAVE_GAS_* to HAVE_AS_*

x86: rename HAVE_GAS_* to HAVE_AS_*

Xen also uses clang's assembler when it is possible. Change the macro
names to not be GAS specific.

Patch produced with:

$ for f in `git grep HAVE_GAS_ | cut -d':' -f1`; \
    do sed -i 's/HAVE_GAS_/HAVE_AS_/g' $f; done

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