x86/clang: allow integrated assembler usage

x86/clang: allow integrated assembler usage

If the required features are present.

Modify as-option-add to add an option in case the test fails, and use
it to detect whether the required clang integrated assembler features
are present.

This patch has been tested with clang 3.5, clang 6, gcc 6.4.0 without
retpoline support and gcc 7.3.1 with retpoline support.

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