cmd/asm,cmd/compile,cmd/internal/obj/riscv: use compressed instructions on riscv64
Make use of compressed instructions on riscv64 - add a compress
pass to the end of the assembler, which replaces non-compressed
instructions with compressed alternatives if possible.
Provide a `compressinstructions` compiler and assembler debug
flag, such that the compression pass can be disabled via
`-asmflags=all=-d=compressinstructions=0` and
`-gcflags=all=-d=compressinstructions=0`. Note that this does
not prevent the explicit use of compressed instructions via
assembly.
Note that this does not make use of compressed control transfer
instructions - this will be implemented in later changes.
Reduces the text size of a hello world binary by ~121KB
and reduces the text size of the go binary on riscv64 by ~1.21MB
(between 8-10% in both cases).
Updates #71105
Cq-Include-Trybots: luci.golang.try:gotip-linux-riscv64
Change-Id: I24258353688554042c2a836deed4830cc673e985
Reviewed-on: https://go-review.googlesource.com/c/go/+/523478 Reviewed-by: Mark Ryan <markdryan@rivosinc.com> Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>