ppc64: Add minimal fallback unwinder.

ppc64: Add minimal fallback unwinder.

This adds a minimal fallback unwinder for ppc64[le] in case we cannot find
CFI for a particular address. It simply always sets the program counter to
the link register, picks the previous stack pointer from the backchain,
and the previous link register from the LR save area.

This is enough for some simple situations when we don't have CFI and
seems to work nicely in the case of perf with libdw powerpc support:
https://lkml.org/lkml/2017/5/18/998

Signed-off-by: Mark Wielaard <mark@klomp.org>