selftests/x86: build fsgsbase_restore.c with clang

When building with clang, via:

    make LLVM=1 -C tools/testing/selftests

Fix this by moving the inline asm to "pure" assembly, in two new files:
clang_helpers_32.S, clang_helpers_64.S.

As a bonus, the pure asm avoids the need for ifdefs, and is now very
simple and easy on the eyes.

Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>