um: Use generic runtime constant implementation

UML should not be using the architecture native runtime constants, since
that requires also having the appropriate instruction fixups (and all
the linker script details).

Not that using that code would be impossible, but it's not worth it.
Just point UML at the generic version.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: e3c92e81 ("runtime constants: add x86 architecture support")
Link: https://lore.kernel.org/all/20240716143644.GA1827132@thelio-3990X/


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>