runtime/cgo: only build xx_cgo_panicmem on iOS

On iOS, when running under lldb, we install xx_cgo_panicmem as
EXC_BAD_ACCESS handler so we can get a proper Go panic for
SIGSEGV. Only build it on iOS.

Updates #38485.

Change-Id: I801c477439e05920a4bb8fdf5eae6f4923ab8274
Reviewed-on: https://go-review.googlesource.com/c/go/+/259440
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/gcc_signal_darwin_nolldb.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !lldb
+// +build !lldb !ios
 // +build darwin
 // +build arm64