mmx: make ldq_u take __m64* directly

mmx: make ldq_u take __m64* directly

Before, if __m64 is allocated in vector or floating-point registers,

	__m64 vs = ldq_u((uint64_t *)src);

would cause src to be loaded into an integer register and then
transferred to an __m64 register. By switching ldq_u's argument type to
__m64 we give the compile enough information to recognize that it can
load to the vector register directly.

This patch is necessary for the Loongson optimizations when __m64 is
typedef'd as double.