cmd/compile: teach regalloc to rightly do nothing on loong64 in case of dynlinking

This is needed before actual support for buildmode=plugin is added.
Should not affect current behavior.

Change-Id: I86371d7e373fd529cb8710850d7b0fbbf1eb52ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/480877
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Meidan Li <[email protected]>
Reviewed-by: abner chenc <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: WANG Xuerui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: xen0n

src/cmd/compile/internal/ssa/regalloc.go

@@ -672,6 +672,8 @@ func (s *regAllocState) init(f *Func) {
 			s.allocatable &^= 1 << 9 // R9
 		case "arm64":
 			// nothing to do
+		case "loong64": // R2 (aka TP) already reserved.
+			// nothing to do
 		case "ppc64le": // R2 already reserved.
 			// nothing to do
 		case "riscv64": // X3 (aka GP) and X4 (aka TP) already reserved.