Implement xfer_partial TARGET_OBJECT_SIGNAL_INFO for NetBSD

NetBSD implements reading and overwriting siginfo_t received by the
tracee. With TARGET_OBJECT_SIGNAL_INFO signal information can be
examined and modified through the special variable $_siginfo.

Implement the "get_siginfo_type" gdbarch method for NetBSD architectures.

As with Linux architectures, cache the created type in the gdbarch when it
is first created.  Currently NetBSD uses an identical siginfo type on
all architectures, so there is no support for architecture-specific fields.

gdb/ChangeLog:

	* nbsd-nat.h (nbsd_nat_target::xfer_partial): New declaration.
	* nbsd-nat.c (nbsd_nat_target::xfer_partial): New function.
	* nbsd-tdep.c (nbsd_gdbarch_data_handle, struct nbsd_gdbarch_data)
	(init_nbsd_gdbarch_data, get_nbsd_gdbarch_data)
	(nbsd_get_siginfo_type): New.
	(nbsd_init_abi): Install gdbarch "get_siginfo_type" method.
	(_initialize_nbsd_tdep): New