runtime: remove thunk.s

Replace with uses of //go:linkname in Go files, direct use of name in .s files.
The only one that really truly needs a jump is reflect.call; the jump is now
next to the runtime.reflectcall assembly implementations.

Change-Id: Ie7ff3020a8f60a8e4c8645fe236e7883a3f23f46
Reviewed-on: https://go-review.googlesource.com/1962
Reviewed-by: Austin Clements <austin@google.com>
diff --git a/src/runtime/iface.go b/src/runtime/iface.go
index f60b6a7..6be1df7 100644
--- a/src/runtime/iface.go
+++ b/src/runtime/iface.go
@@ -386,6 +386,7 @@
 	return
 }
 
+//go:linkname reflect_ifaceE2I reflect.ifaceE2I
 func reflect_ifaceE2I(inter *interfacetype, e interface{}, dst *fInterface) {
 	*dst = assertE2I(inter, e)
 }