runtime: handle locked mmap failure on Linux

Used to then die on a nil pointer situation. Most Linux standard setups are rather
restrictive regarding the default amount of lockable memory.

R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6997049

src/pkg/runtime/mem_linux.c

@@ -10,6 +10,7 @@
 
 enum
 {
+	EAGAIN = 11,
 	ENOMEM = 12,
 	_PAGE_SIZE = 4096,
 };
@@ -63,6 +64,10 @@ runtime·SysAlloc(uintptr n)
 			runtime·printf("if you're running SELinux, enable execmem for this process.\n");
 			runtime·exit(2);
 		}
+		if(p == (void*)EAGAIN) {
+			runtime·printf("runtime: mmap: too much locked memory (check 'ulimit -l').\n");
+			runtime·exit(2);
+		}
 		return nil;
 	}
 	return p;