runtime: Do not reserve huge amount of swap on 32 bit architectures.

The mmap() call which reserves the arena should have MAP_NORESERVE
flag as in typical cases this memory will never be (fully) needed.
This matters in environments which do not do Linux style memory
overcommit, such as OpenIndiana/OpenSolaris/Solaris.

The MAP_NORESERVE flag does not exist on all operating systems
(for example FreeBSD). Therefore we define it to zero value in
case it does not exist.

Fixes issue 21.

From-SVN: r196088

libgo/runtime/mem.c

@@ -18,6 +18,10 @@
 #endif
 #endif
 
+#ifndef MAP_NORESERVE
+#define MAP_NORESERVE 0
+#endif
+
 #ifdef USE_DEV_ZERO
 static int dev_zero = -1;
 #endif
@@ -134,7 +138,11 @@ runtime_SysReserve(void *v, uintptr n)
 		return v;
 	}
 	
-	p = runtime_mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, fd, 0);
+	// Use the MAP_NORESERVE mmap() flag here because typically most of
+	// this reservation will never be used. It does not make sense
+	// reserve a huge amount of unneeded swap space. This is important on
+	// systems which do not overcommit memory by default.
+	p = runtime_mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_NORESERVE, fd, 0);
 	if(p == MAP_FAILED)
 		return nil;
 	return p;