cmd/compile: instrinsify math/bits.Mul on ppc64x

Add SSA rules to intrinsify Mul/Mul64 on ppc64x.

benchmark             old ns/op     new ns/op     delta
BenchmarkMul-40       8.80          0.93          -89.43%
BenchmarkMul32-40     1.39          1.39          +0.00%
BenchmarkMul64-40     5.39          0.93          -82.75%

Updates #24813

Change-Id: I6e95bfbe976a2278bd17799df184a7fbc0e57829
Reviewed-on: https://go-review.googlesource.com/138917


Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>