Add a signal frame unwinder for FreeBSD/mipsn32.

Add a signal frame unwinder for FreeBSD/mipsn32.

The N32 signal frame uses an identical layout to N64, so reuse the N64
handler.  The N32 signal trampoline does use one different instruction
relative to N64, so a separate tramp_frame is required.

gdb/ChangeLog:

	* mips-fbsd-tdep.c (MIPS_INST_ADDIU_A0_SP_N32): Define.
	(mipsn32_fbsd_sigframe): Define.
	(mips_fbsd_init_abi): Install mipsn32_fbsd_sigframe unwinder
	for FreeBSD/mipsn32.