Implement the "get_siginfo_type" gdbarch method for FreeBSD architectures.

Implement the "get_siginfo_type" gdbarch method for FreeBSD architectures.

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

gdb/ChangeLog:

	* fbsd-tdep.c (fbsd_gdbarch_data_handle, struct fbsd_gdbarch_data)
	(init_fbsd_gdbarch_data, get_fbsd_gdbarch_data)
	(fbsd_get_siginfo_type): New.
	(fbsd_init_abi): Install gdbarch "get_siginfo_type" method.
	(_initialize_fbsd_tdep): New.