x86/efi: enable MS ABI attribute on clang

x86/efi: enable MS ABI attribute on clang

Or else the EFI service calls will use the wrong calling convention.

The __ms_abi__ attribute is available on all supported versions of
clang. Add a specific Clang check because the GCC version reported by
Clang is below the required 4.4 to use the __ms_abi__ attribute.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
master commit: 92f5ffa58d188c9f9a9f1bcdccb6d6348d9df612
master date: 2021-02-04 14:02:32 +0100