selftests/x86: Use __builtin_ia32_read/writeeflags

The asm to read and write EFLAGS from userspace is horrible.  The
compiler builtins are now available on all supported compilers, so
use them instead.

(The compiler builtins are also unnecessarily ugly, but that's a
 more manageable level of ugliness.)

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/aee4b1cdfc56083eb779ce927b7d3459aad2af76.1604346818.git.luto@kernel.org