runtime: set up g early

runtime.throw needs a g to work properly. Set up g early, to
    ensure that if something goes wrong in the runtime startup (e.g.
    runtime.check fails), the program terminates in a reasonable way.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176657

From-SVN: r271088

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-76ab85364745e445498fe53f9ca8e37b49650779
+5c2c4743980556c041561533ef31762f524737ca
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -18,6 +18,7 @@ import (
 //go:linkname acquirep runtime.acquirep
 //go:linkname releasep runtime.releasep
 //go:linkname incidlelocked runtime.incidlelocked
+//go:linkname ginit runtime.ginit
 //go:linkname schedinit runtime.schedinit
 //go:linkname ready runtime.ready
 //go:linkname stopm runtime.stopm
@@ -515,6 +516,15 @@ func cpuinit() {
 	cpu.Initialize(env)
 }
 
+func ginit() {
+	_m_ := &m0
+	_g_ := &g0
+	_m_.g0 = _g_
+	_m_.curg = _g_
+	_g_.m = _m_
+	setg(_g_)
+}
+
 // The bootstrap sequence is:
 //
 //	call osinit
@@ -524,13 +534,7 @@ func cpuinit() {
 //
 // The new G calls runtime·main.
 func schedinit() {
-	_m_ := &m0
+	_g_ := getg()
-	_g_ := &g0
-	_m_.g0 = _g_
-	_m_.curg = _g_
-	_g_.m = _m_
-	setg(_g_)
-
 	sched.maxmcount = 10000
 
 	usestackmaps = probestackmaps()

libgo/runtime/go-libmain.c

@@ -225,6 +225,7 @@ gostart (void *arg)
     return NULL;
   runtime_isstarted = true;
 
+  runtime_ginit ();
   runtime_check ();
   runtime_args (a->argc, (byte **) a->argv);
   setncpu (getproccount ());

libgo/runtime/go-main.c

@@ -48,6 +48,7 @@ main (int argc, char **argv)
     setIsCgo ();
 
   __go_end = (uintptr)_end;
+  runtime_ginit ();
   runtime_cpuinit ();
   runtime_check ();
   runtime_args (argc, (byte **) argv);

libgo/runtime/runtime.h

@@ -240,6 +240,8 @@ int32	runtime_snprintf(byte*, int32, const char*, ...);
 #define runtime_memmove(a, b, s) __builtin_memmove((a), (b), (s))
 String	runtime_gostringnocopy(const byte*)
   __asm__ (GOSYM_PREFIX "runtime.gostringnocopy");
+void	runtime_ginit(void)
+  __asm__ (GOSYM_PREFIX "runtime.ginit");
 void	runtime_schedinit(void)
   __asm__ (GOSYM_PREFIX "runtime.schedinit");
 void	runtime_initsig(bool)