Fetch signal information for native FreeBSD processes.

Use the `pl_siginfo' field in the `struct ptrace_lwpinfo' object returned
by the PT_LWPINFO ptrace() request to supply the current contents of
$_siginfo for each thread.  Note that FreeBSD does not supply a way to
modify the signal information for a thread, so $_siginfo is read-only for
FreeBSD.

To handle 32-bit processes on a 64-bit host, define types for 32-bit
compatible siginfo_t and convert the 64-bit siginfo_t to the 32-bit
equivalent when supplying information for a 32-bit process.

gdb/ChangeLog:

	* fbsd-nat.c [PT_LWPINFO && __LP64__] (union sigval32)
	(struct siginfo32): New.
	[PT_LWPINFO] (fbsd_siginfo_size, fbsd_convert_siginfo): New.
	(fbsd_xfer_partial) [PT_LWPINFO]: Handle TARGET_OBJECT_SIGNAL_INFO
	via ptrace(PT_LWPINFO).