Use gdbarch_long_bit to determine layout of FreeBSD siginfo_t.

Use gdbarch_long_bit to determine layout of FreeBSD siginfo_t.

FreeBSD architectures are either ILP32 or LP64 resulting in two
different layouts for siginfo_t.  Previously, the 'bits_per_word'
member of bfd_arch_info was used to determine the layout to use for a
given FreeBSD architecture.  However, mipsn32 architectures inherit
from a 64-bit mips architecture where bits_per_word is 64.  As a
result, $_siginfo was not properly extracted from FreeBSD/mipsn32 core
dumps.  Fix this by using gdbarch_long_bit instead of 'bits_per_word'
to determine if a FreeBSD architecture is ILP32 or LP64.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_siginfo_size): Use gdbarch_long_bit.
	(fbsd_convert_siginfo): Likewise.
	* fbsd-tdep.c (fbsd_core_xfer_siginfo): Likewise.