Fix undefined function errors on PPC and MIPS.

undefined_tramp hardcodes the register in which FDEFN resides, but the
format was recently changed (f69e89d..).

Other platforms can be susceptible to this.
A proper fix would avoid hardcoding this by exporting
sc-offset-scn-byte/sc-offset-offset-byte, and register offsets.

Thanks to the GCC Compile Farm project for providing machines for
testing and uncovering this.