runtime: keep build version around in binaries

So we can tell from a binary which version of
Go built it.

LGTM=minux, rsc
R=golang-codereviews, minux, khr, rsc, dave
CC=golang-codereviews
https://golang.org/cl/117040043

src/cmd/dist/buildruntime.c

@@ -30,7 +30,8 @@ mkzversion(char *dir, char *file)
 		"package runtime\n"
 		"\n"
 		"const defaultGoroot = `%s`\n"
-		"const theVersion = `%s`\n", goroot_final, goversion));
+		"const theVersion = `%s`\n"
+		"var buildVersion = theVersion\n", goroot_final, goversion));
 
 	writefile(&out, file, 0);
 	

src/pkg/runtime/proc.c

@@ -129,6 +129,8 @@ static bool exitsyscallfast(void);
 static bool haveexperiment(int8*);
 static void allgadd(G*);
 
+extern String runtime·buildVersion;
+
 // The bootstrap sequence is:
 //
 //	call osinit
@@ -188,6 +190,13 @@ runtime·schedinit(void)
 		runtime·copystack = false;
 
 	mstats.enablegc = 1;
+
+	if(runtime·buildVersion.str == nil) {
+		// Condition should never trigger.  This code just serves
+		// to ensure runtime·buildVersion is kept in the resulting binary.
+		runtime·buildVersion.str = (uint8*)"unknown";
+		runtime·buildVersion.len = 7;
+	}
 }
 
 extern void main·init(void);