RISC-V: gp_in_global needs register keyword

The Intel kernel build robot recently pointed out that I missed the
register keyword on this one when I refactored the code to remove local
register variables (which aren't supported by LLVM).  GCC's manual
indicates that global register variables must have the register keyword,
As far as I can tell lacking the register keyword causes GCC to ignore
the __asm__ and treat this as a regular variable, but I'm not sure how
that didn't show up as some sort of failure.

Fixes: 52e7c52d ("RISC-V: Stop relying on GCC's register allocator's hueristics")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>