cmd/internal/obj/riscv,internal/bytealg: synthesize MIN/MAX/MINU/MAXU instructions

cmd/internal/obj/riscv,internal/bytealg: synthesize MIN/MAX/MINU/MAXU instructions

Provide a synthesized version of the MIN/MAX/MINU/MAXU instructions
if they're not natively available. This allows these instructions to
be used in assembly unconditionally.

Use MIN in internal/bytealg.compare.

Cq-Include-Trybots: luci.golang.try:gotip-linux-riscv64
Change-Id: I8a5a3a59f0a9205e136fc3d673b23eaf3ca469f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/653295
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>