Following on from the FRV GNU/Linux sigtramp code (which is clearly a 
clone of the m68k code, even down to the comments), this moves the m68k 
GNU/Linux signal trampoline code to the file m68klinux-tdep.c and then 
tweaks it so it builds (by using trad-frame).

This in turn lets us eliminate another DEPRECATED_PC_IN_SIGTRAMP call.

committed,
Andrew