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/bootstrap.zig

@@ -1,8 +1,9 @@
 // This file is in a package which has the root source file exposed as "@root".
 
 const root = @import("@root");
-const linux = @import("linux.zig");
-const cstr = @import("cstr.zig");
+const std = @import("std");
+const linux = std.linux;
+const cstr = std.cstr;
 
 const want_start_symbol = switch(@compileVar("os")) {
     Os.linux => true,