runtime: call symtabinit earlier

Otherwise, we won't get a stack trace in some of the early init.

Here's one example:

        http://build.golang.org/log/a96d10f6aee1fa3e3ae51f41da46d414a7ab02de

After walking the stack by hand in acid, I was able to determine
that the stackalloc inside mpreinit was causing the throw.

LGTM=rsc
R=rsc, dvyukov
CC=golang-codereviews
https://golang.org/cl/72450044

src/pkg/runtime/proc.c

@@ -146,6 +146,7 @@ runtime·schedinit(void)
 	runtime·sched.maxmcount = 10000;
 	runtime·precisestack = true; // haveexperiment("precisestack");
 
+	runtime·symtabinit();
 	runtime·mallocinit();
 	mcommoninit(m);
 	
@@ -159,9 +160,6 @@ runtime·schedinit(void)
 	runtime·goenvs();
 	runtime·parsedebugvars();
 
-	// Allocate internal symbol table representation now, we need it for GC anyway.
-	runtime·symtabinit();
-
 	runtime·sched.lastpoll = runtime·nanotime();
 	procs = 1;
 	p = runtime·getenv("GOMAXPROCS");