We don't want to leave a partial assembly output if the compiler
fails during compilation.

Technically speaking, there's still a risk of leaving a partially-
written output file if the compiler dies during file copy. To fix
that, we have to write to a temporary file in the same filesystem as
the final output file and rename the temporary file to atomically
replace the output file. We don't do that in this patch for the sake
of succinctness, though.

With these directives, gdb can print out an error location when
a compiled program crashes.

No functional change

Now, chibicc can print out an error message with an error location
like this:

  $ ./chibicc 1+foo
  1+foo
    ^ expected a number