Make PrologEpilogInserter save/restore all callee saved registers in
functions which call __builtin_unwind_init()
__builtin_unwind_init() is an undocumented gcc intrinsic which has
this effect, and is used in libgcc_eh.
Goes part of the way toward fixing PR8541.
This obsoletes the ugly hack to libgcc's unwind code from r245272, and
should also work for other arches, so revert the hack too.
707d0cef66