runtime/cgo: define x_cgo_inittls() for android/arm64.

On android, runtime.tls_g is a normal variable.
TLS offset is computed in x_cgo_inittls.

Change-Id: I64cfd3543040776dcdf73cad8dba54fc6aaf6f35
Reviewed-on: https://go-review.googlesource.com/17245Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/cgo/gcc_android_arm64.c

+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include <pthread.h>
+#include <signal.h>
+#include <stdio.h>
+#include <sys/limits.h>
+#include "libcgo.h"
+
+#define magic1 (0x23581321345589ULL)
+
+// inittls allocates a thread-local storage slot for g.
+//
+// It finds the first available slot using pthread_key_create and uses
+// it as the offset value for runtime.tlsg.
+static void
+inittls(void **tlsg, void **tlsbase)
+{
+	pthread_key_t k;
+	int i, err;
+
+	err = pthread_key_create(&k, nil);
+	if(err != 0) {
+		fatalf("pthread_key_create failed: %d", err);
+	}
+	pthread_setspecific(k, (void*)magic1);
+	for (i=0; i<PTHREAD_KEYS_MAX; i++) {
+		if (*(tlsbase+i) == (void*)magic1) {
+			*tlsg = (void*)(i*sizeof(void *));
+			pthread_setspecific(k, 0);
+			return;
+		}
+	}
+	fatalf("could not find pthread key");
+}
+
+void (*x_cgo_inittls)(void **tlsg, void **tlsbase) = inittls;