cmd/internal/objabi: add regabi GOEXPERIMENT

This is the "feature flag" for the register calling convention work
(though since this work is expected to extend over a few releases,
it's not version-prefixed). This will let us develop the register
calling convention on the main branch while maintaining an easy toggle
between the old and new ABIs.

Updates #40724.

Change-Id: I129c8d87d34e6fa0910b6fa43efb35b706021637
Reviewed-on: https://go-review.googlesource.com/c/go/+/252257
Reviewed-by: Cherry Zhang <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: aclements

src/cmd/internal/objabi/util.go

@@ -131,6 +131,11 @@ func init() {
 			addexp(f)
 		}
 	}
+
+	// regabi is only supported on amd64.
+	if GOARCH != "amd64" {
+		Regabi_enabled = 0
+	}
 }
 
 // Note: must agree with runtime.framepointer_enabled.
@@ -161,6 +166,7 @@ var (
 	Fieldtrack_enabled        int
 	Preemptibleloops_enabled  int
 	Staticlockranking_enabled int
+	Regabi_enabled            int
 )
 
 // Toolchain experiments.
@@ -174,6 +180,7 @@ var exper = []struct {
 	{"fieldtrack", &Fieldtrack_enabled},
 	{"preemptibleloops", &Preemptibleloops_enabled},
 	{"staticlockranking", &Staticlockranking_enabled},
+	{"regabi", &Regabi_enabled},
 }
 
 var defaultExpstring = Expstring()