Constify target_pass_signals and target_program_signals

Constify target_pass_signals and target_program_signals

This constifies the final parameter to target_pass_signals and
target_program_signals and updates the rest of gdb.

Note that I have no way to test the nto-procfs.c change.

gdb/ChangeLog
2019-01-14  Tom Tromey  <tom@tromey.com>

	* target-debug.h (target_debug_print_signals): Constify.
	* nto-procfs.c (nto_procfs_target::pass_signals): Update.
	* procfs.c (procfs_target::pass_signals): Update.
	* linux-nat.c (linux_nat_target::pass_signals): Update.
	* linux-nat.h (class linux_nat_target) <pass_signals>: Update.
	* target-delegates.c: Rebuild.
	* remote.c (remote_target::program_signals): Update.
	(remote_target::pass_signals): Update.
	* target.c (target_pass_signals): Constify argument.
	(target_program_signals): Likewise.
	* target.h (struct target_ops) <pass_signals, program_signals>:
	Constify argument.
	(target_pass_signals, target_program_signals): Constify argument.