* 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