mips: Unconditionally select ARCH_HAS_CURRENT_STACK_POINTER

Now that the minimum supported version of LLVM for building the kernel
has been bumped to 15.0.0, the selection of
ARCH_HAS_CURRENT_STACK_POINTER can be made unconditional since it is
always true.

Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250821-bump-min-llvm-ver-15-v2-5-635f3294e5f0@kernel.org


Signed-off-by: Nathan Chancellor <nathan@kernel.org>