8087120: [GCC5] java.lang.StackOverflowError on Zero JVM initialization on non x86 platforms

Use __builtin_frame_address(0) rather than returning address of local variable.

Reviewed-by: dholmes

hotspot/src/os_cpu/linux_zero/vm/os_linux_zero.cpp

@@ -59,8 +59,8 @@
 extern sigjmp_buf* get_jmp_buf_for_continuation();
 
 address os::current_stack_pointer() {
-  address dummy = (address) &dummy;
-  return dummy;
+  // return the address of the current function
+  return (address)__builtin_frame_address(0);
 }
 
 frame os::get_sender_for_C_frame(frame* fr) {