riscv: s64ilp32: Introduce PTR_L and PTR_S

REG_L and REG_S can't satisfy s64ilp32 situation, because its
__SIZEOF_POINTER__*8 != __riscv_xlen. So we introduce new PTR_L
and PTR_S macro to help head.S and entry.S deal with the pointer
data type and replace all REG_L/S by PTR_L/S to fit the current
algorithm in memcpy, memove, memset, strcmp, strlen and strncmp.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>