runtime: fix bug in GOTRACEBACK=crash causing suppression of core dumps.

Because gotraceback is called early and often, its cache commits to the value of getenv("GOTRACEBACK") before getenv is even ready.  So now we reset its cache once getenv becomes ready.  Panicking programs now dump core again.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/97800045

src/pkg/runtime/runtime.c

@@ -11,6 +11,13 @@ enum {
 	maxround = sizeof(uintptr),
 };
 
+// Keep a cached value to make gotraceback fast,
+// since we call it on every call to gentraceback.
+// The cached value is a uint32 in which the low bit
+// is the "crash" setting and the top 31 bits are the
+// gotraceback value.
+static uint32 traceback_cache = ~(uint32)0;
+
 // The GOTRACEBACK environment variable controls the
 // behavior of a Go program that is crashing and exiting.
 //	GOTRACEBACK=0   suppress all tracebacks
@@ -20,12 +27,6 @@ enum {
 int32
 runtime·gotraceback(bool *crash)
 {
-	// Keep a cached value to make gotraceback fast,
-	// since we call it on every call to gentraceback.
-	// The cached value is a uint32 in which the low bit
-	// is the "crash" setting and the top 31 bits are the
-	// gotraceback value.
-	static uint32 cache = ~(uint32)0;
 	byte *p;
 	uint32 x;
 
@@ -33,7 +34,7 @@ runtime·gotraceback(bool *crash)
 		*crash = false;
 	if(m->traceback != 0)
 		return m->traceback;
-	x = runtime·atomicload(&cache);
+	x = runtime·atomicload(&traceback_cache);
 	if(x == ~(uint32)0) {
 		p = runtime·getenv("GOTRACEBACK");
 		if(p == nil)
@@ -44,7 +45,7 @@ runtime·gotraceback(bool *crash)
 			x = (2<<1) | 1;
 		else
 			x = runtime·atoi(p)<<1;	
-		runtime·atomicstore(&cache, x);
+		runtime·atomicstore(&traceback_cache, x);
 	}
 	if(crash != nil)
 		*crash = x&1;
@@ -137,6 +138,8 @@ runtime·goenvs_unix(void)
 	syscall·envs.array = (byte*)s;
 	syscall·envs.len = n;
 	syscall·envs.cap = n;
+
+	traceback_cache = ~(uint32)0;
 }
 
 int32