improved behavior on debug safety crash

* instead of emitting a breakpoint for a debug safety crash,
   zig calls a panic function which prints an error message
   and a stack trace and then calls abort.
 * on freestanding OS, this panic function has a default
   implementation of a simple infinite loop.
 * users can override the panic implementation by providing
   `pub fn panic(message: []const u8) -> unreachable { }`
 * workaround for LLVM segfaulting when you try to use cold
   calling convention on ARM.

closes #245

std/debug.zig

@@ -42,6 +42,8 @@ pub fn writeStackTrace(out_stream: &io.OutStream) -> %void {
             st.debug_str = (%return st.elf.findSection(".debug_str")) ?? return error.MissingDebugInfo;
             %return scanAllCompileUnits(st);
 
+            %return out_stream.printf("(...work-in-progress stack unwinding code follows...)\n");
+
             var maybe_fp: ?&const u8 = @frameAddress();
             while (true) {
                 const fp = maybe_fp ?? break;