cmd/{asm,compile}: add fused multiply-add support on riscv64

cmd/{asm,compile}: add fused multiply-add support on riscv64

Add support to the assembler for F[N]M{ADD,SUB}[SD] instructions.
Argument order is:

  OP RS1, RS2, RS3, RD

Also, add support for the FMA intrinsic to the compiler. Automatic
FMA matching is left to a future CL.

Change-Id: I47166c7393b2ab6bfc2e42aa8c1a8997c3a071b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/293030
Trust: Michael Munday <mike.munday@lowrisc.org>
Run-TryBot: Michael Munday <mike.munday@lowrisc.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>