[GDB] Hurd: Simplify the reply_mig_hack.awk script.

[GDB] Hurd: Simplify the reply_mig_hack.awk script.

	gdb/
	* reply_mig_hack.awk: Don't bother to declare an intermediate
	function pointer variable.

... allowing us to simplify the parsing a little bit.  And, instead of
"warning: initialization from incompatible pointer type", we now get "warning:
function called through a non-compatible type".  Oh well.