m68k_sigtramp_saved_pc() - like the comon sigtramp_saved_pc, but
uses SIG_PC_FP_OFFSET to find the saved PC (used for SunOS).
nbsd_in_sigtramp() - determine whether a PC value is within the
signal trampoline, but do so WITHOUT hard-coding the address
of the end of the stack (uses "__ps_strings" at run time).
This allows a common gdb to work on all m68k. (yea!)