Remove shadowing from darwin-nat.c

Building on Darwin with gcc showed that darwin-nat.c had some
shadowing variable declarations.  This removes them.

gdb/ChangeLog
2019-06-11  Tom Tromey  <tromey@adacore.com>

	* darwin-nat.c (darwin_decode_exception_message)
	(darwin_decode_message, darwin_nat_target::kill): Fix shadowing.