selftests/x86: build sysret_rip.c with clang

When building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...the build fails because clang's inline asm doesn't support all of the
features that are used in the asm() snippet in sysret_rip.c.

Fix this by moving the asm code into the clang_helpers_64.S file, where
it can be built with the assembler's full set of features.

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>