MIPS: Support '%w' token in inline asm template for MSA (#91920)

MSA registers share the FPRs as its bottom half. So that we can use MSA
instructions to work with normal float/double:
   double a, b, c;
   asm volatile ("fmadd.d %w0, %w1, %w2" : "+f"(a) : "f"(b), "f"(c));

GCC has support it for quite long time.