go.tools/ssa/interp: fix build (add missing runtime function)

Introduced by CL 41860043.

R=adonovan
TBR=adonovan
CC=golang-dev
https://golang.org/cl/42950044

ssa/interp/external.go

@@ -88,6 +88,7 @@ var externals = map[string]externalFn{
 	"runtime.SetFinalizer":            ext۰runtime۰SetFinalizer,
 	"runtime.getgoroot":               ext۰runtime۰getgoroot,
 	"strings.IndexByte":               ext۰strings۰IndexByte,
+	"sync.runtime_registerPool":       ext۰sync۰runtime_registerPool,
 	"sync.runtime_Syncsemcheck":       ext۰sync۰runtime_Syncsemcheck,
 	"sync/atomic.AddInt32":            ext۰atomic۰AddInt32,
 	"sync/atomic.CompareAndSwapInt32": ext۰atomic۰CompareAndSwapInt32,
@@ -226,6 +227,10 @@ func ext۰strings۰IndexByte(fn *ssa.Function, args []value) value {
 	return -1
 }
 
+func ext۰sync۰runtime_registerPool(fn *ssa.Function, args []value) value {
+	return nil
+}
+
 func ext۰sync۰runtime_Syncsemcheck(fn *ssa.Function, args []value) value {
 	return nil
 }