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/runtime.go b/src/runtime/runtime.go
index 2e291c2..2ce4618 100644
--- a/src/runtime/runtime.go
+++ b/src/runtime/runtime.go
@@ -4,6 +4,8 @@
 
 package runtime
 
+import _ "unsafe" // for go:linkname
+
 //go:generate go run wincallback.go
 
 var ticks struct {
@@ -55,8 +57,8 @@
 var envs []string
 var argslice []string
 
-// called from syscall
-func runtime_envs() []string { return envs }
+//go:linkname syscall_runtime_envs syscall.runtime_envs
+func syscall_runtime_envs() []string { return envs }
 
-// called from os
-func runtime_args() []string { return argslice }
+//go:linkname os_runtime_args os.runtime_args
+func os_runtime_args() []string { return argslice }