Switch from __ABICALLS__ to __mips_abicalls like upstream GCC does in
the generic MIPS target logic.

force 2nd to argument to unsigned byte value.
(found by t_strchr and t_strrchr tests)

Make these 64-bit clean.

Add asm files for str(r)chr by copying the files for (r)index from
src/libc/arch/mips/string.
Add XLEAF() for index entry points.