cmd/internal/obj/riscv: use native rotation instructions for rva22u64

When rva22u64 is available, we can now use the native rotation instructions
from the Zbb extension. Use these instead of synthesising rotation
instructions.

This provides a significant performance gain for SHA-512, the following
benchmarked on a StarFive VisionFive 2:

                    │ sha512.rva20u64 │            sha512.rva22u64            │
                    │       B/s       │      B/s       vs base                │
Hash8Bytes/New-4         859.4Ki ± 0%   1337.9Ki ± 0%  +55.68% (p=0.000 n=10)
Hash8Bytes/Sum384-4      888.7Ki ± 1%   1308.6Ki ± 1%  +47.25% (p=0.000 n=10)
Hash8Bytes/Sum512-4      869.1Ki ± 0%   1269.5Ki ± 1%  +46.07% (p=0.000 n=10)
Hash1K/New-4             19.83Mi ± 0%    29.03Mi ± 0%  +46.38% (p=0.000 n=10)
Hash1K/Sum384-4          20.00Mi ± 0%    28.86Mi ± 0%  +44.30% (p=0.000 n=10)
Hash1K/Sum512-4          19.93Mi ± 0%    28.72Mi ± 0%  +44.11% (p=0.000 n=10)
Hash8K/New-4             23.85Mi ± 0%    34.12Mi ± 0%  +43.09% (p=0.000 n=10)
Hash8K/Sum384-4          23.88Mi ± 0%    34.09Mi ± 0%  +42.77% (p=0.000 n=10)
Hash8K/Sum512-4          23.87Mi ± 0%    34.07Mi ± 0%  +42.71% (p=0.000 n=10)
geomean                  7.399Mi         10.78Mi       +45.77%

Change-Id: I9dca8e3f311eea101684c806cb998872dc697288
Reviewed-on: https://go-review.googlesource.com/c/go/+/572716
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: M Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>

src/cmd/internal/obj/riscv/obj.go

@@ -2291,6 +2291,11 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
 
 // instructionsForRotate returns the machine instructions for a bitwise rotation.
 func instructionsForRotate(p *obj.Prog, ins *instruction) []*instruction {
+	if buildcfg.GORISCV64 >= 22 {
+		// Rotation instructions are supported natively.
+		return []*instruction{ins}
+	}
+
 	switch ins.as {
 	case AROL, AROLW, AROR, ARORW:
 		// ROL -> OR (SLL x y) (SRL x (NEG y))