Fix up a couple oddities in GDB's signal names and strings.

Fix up a couple oddities in GDB's signal names and strings.

- The Mach exception/signals escaped the TARGET_ -> GDB_ prefix change
  done a while ago, but there's no real reason for that.  I grepped
  for TARGET_EXC and fixed all found, which unsurprisingly, means
  darwin-nat.c needed fixing.  I think the change there is as obvious
  and trivial as it can get, so I'd be quite surprised if this broke
  anything there somehow.

- GDB_SIGNAL_LAST's description string was unnecessarily inconsistent
  with the enum name.

Built on x86_64 Fedora 17.

gdb/
2013-10-22  Pedro Alves  <palves@redhat.com>

	* include/gdb/signals.def (TARGET_EXC_BAD_ACCESS): Rename to
	GDB_EXC_BAD_ACCESS.
	(TARGET_EXC_BAD_INSTRUCTION): Rename to GDB_EXC_BAD_INSTRUCTION.
	(TARGET_EXC_ARITHMETIC): Rename to GDB_EXC_ARITHMETIC.
	(TARGET_EXC_EMULATION): Rename to GDB_EXC_EMULATION.
	(TARGET_EXC_SOFTWARE): Rename to GDB_EXC_SOFTWARE.
	(TARGET_EXC_BREAKPOINT): Rename to GDB_EXC_BREAKPOINT.
	(GDB_SIGNAL_LAST): Change description string.
	* common/signals.c (gdb_signal_from_host, do_gdb_signal_to_host):
	Adjust to signal renaming.
	* darwin-nat.c (darwin_decode_message): Likewise.