Signal unwinder for mips-irix N32.

    Signal unwinder for mips-irix N32.
        * mips-irix-tdep.c: #include mips-tdep.h, trad-frame.h, and
        tramp-frame.h.
        (SIGTRAMP_FRAME_SIZE, SIGCONTEXT_PC_OFF, SIGCONTEXT_REGS_OFF)
        (SIGCONTEXT_FPREGS_OFF, SIGCONTEXT_FPCSR_OFF, SIGCONTEXT_HI_OFF)
        (SIGCONTEXT_LO_OFF): New macros.
        (mips_irix_n32_tramp_frame_init): New function.
        (mips_irix_n32_tramp_frame): New static constant.
        (mips_irix_init_abi): Prepend the mips_irix_n32_tramp_frame unwinder.