cmd/go: add rva23u64 as a valid value for GORISCV64

The RVA23 profile was ratified on the 21st of October 2024.

https://riscv.org/announcements/2024/10/risc-v-announces-ratification-of-the-rva23-profile-standard/

Now that it's ratified we can add rva23u64 as a valid value for the
GORISCV64 environment variable. This will allow the compiler and
assembler to generate instructions made mandatory by the new profile
without a runtime check.  Examples of such instructions include those
introduced by the Vector and Zicond extensions.

Setting GORISCV64=rva23u64 defines the riscv64.rva20u64,
riscv64.rva22u64 and riscv64.rva23u64 build tags, sets the internal
variable buildcfg.GORISCV64 to 23 and defines the macros
GORISCV64_rva23u64, hasV, hasZba, hasZbb, hasZbs, hasZfa, and
hasZicond for use in assembly language code.

Updates #61476

Change-Id: I7641c23084fa52891c9a18df58f4013cb6597d88
Reviewed-on: https://go-review.googlesource.com/c/go/+/633417
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>

src/cmd/internal/testdir/testdir_test.go

@@ -1489,7 +1489,7 @@ var (
 		"ppc64x":  {}, // A pseudo-arch representing both ppc64 and ppc64le
 		"s390x":   {},
 		"wasm":    {},
-		"riscv64": {"GORISCV64", "rva20u64", "rva22u64"},
+		"riscv64": {"GORISCV64", "rva20u64", "rva22u64", "rva23u64"},
 	}
 )