gdb, gdbserver: make status_to_str return std::string

Instead of using a static buffer.  This is safer, and we don't really
mind about any extra dynamic allocation here, since it's only used for
debug purposes.

gdb/ChangeLog:

	* nat/linux-waitpid.c (status_to_str): Return std::string.
	* nat/linux-waitpid.h (status_to_str): Likewise.
	* linux-nat.c (linux_nat_post_attach_wait): Adjust.
	(linux_nat_target::attach): Adjust.
	(linux_handle_extended_wait): Adjust.
	(wait_lwp): Adjust.
	(stop_wait_callback): Adjust.
	(linux_nat_filter_event): Adjust.
	(linux_nat_wait_1): Adjust.
	* nat/linux-waitpid.c (status_to_str): Adjust.
	* nat/linux-waitpid.h (status_to_str): Adjust.

gdbserver/ChangeLog:

	* linux-low.cc (linux_process_target::wait_for_event_filtered):
	Adjust to status_to_str returning std::string.

Change-Id: Ia8aead70270438a5690f243e6faafff6c38ff757