runtime: revert 6974:1f3c3696babb

I missed that environment is used during runtime setup,
well before go init() functions run. Implemented os-dependent
runtime.goenvs functions to allow for different unix, plan9 and
windows versions of environment discovery.

R=rsc, paulzhol
CC=golang-dev
https://golang.org/cl/3787046

src/pkg/runtime/linux/thread.c

@@ -263,6 +263,12 @@ runtime·osinit(void)
 {
 }
 
+void
+runtime·goenvs(void)
+{
+	runtime·goenvs_unix();
+}
+
 // Called to initialize a new m (including the bootstrap m).
 void
 runtime·minit(void)