Do we still care about compilers without __seg_fs and __seg_gs support??

["H. Peter Anvin" <hpa@zytor.com>]

As of Linux 6.16, we require:

	gcc 8.1 or higher
	clang 15.0.0 or higher

If my reading of the release notes is correct, then both versions *should*
supported __seg_fs and __seg_gs, but we have:

config CC_HAS_NAMED_AS
        def_bool $(success,echo 'int __seg_fs fs; int __seg_gs gs;' | $(CC) -x
c - -S -o /dev/null)
        depends on CC_IS_GCC

We don't even try on clang.

Being able to actually rely on the compiler for this would make a lot of
things cleaner.  For one thing, I'm trying to untangle a bunch of ugliness in
the code sharing between realmode and proper flat mode code...

Uros, you seem to have touched this code as recently as earlier this year; any
thoughts?

What about the LLVM people, any insights?

	-hpa