[libunwind] Define _Unwind_Backtrace for powerpc, sparc

Add SPARC to the list of platforms for which we provide a full
unwind implementation which leads to _Unwind_Backtrace being defined within
libunwind.so.

Likewise for PPC (see D118320 for background).

Reviewed By: #libunwind, MaskRay, Arfrever

Differential Revision: https://reviews.llvm.org/D119068