runtime: correct ptrSize test in Linux version of sysReserve

Change-Id: I90a8ca51269528a307e0d6f52436fc7913cd7900
Reviewed-on: https://go-review.googlesource.com/1541Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mem_linux.go

@@ -88,7 +88,7 @@ func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
 	// much address space.  Instead, assume that the reservation is okay
 	// if we can reserve at least 64K and check the assumption in SysMap.
 	// Only user-mode Linux (UML) rejects these requests.
-	if ptrSize == 7 && uint64(n) > 1<<32 {
+	if ptrSize == 8 && uint64(n) > 1<<32 {
 		p := mmap_fixed(v, 64<<10, _PROT_NONE, _MAP_ANON|_MAP_PRIVATE, -1, 0)
 		if p != v {
 			if uintptr(p) >= 4096 {