Update libunwind configuration check to look for unw_backtrace().

Update libunwind configuration check to look for unw_backtrace(), which
is a newer API not available in older versions of libunwind.
Simplify backtracing.

Simplify backtracing to not ignore any frames, and compensate for this
in pprof in order to increase flexibility with respect to function-based
refactoring even in the presence of non-deterministic inlining.  Modify
pprof to blacklist all jemalloc allocation entry points including
non-standard ones like mallocx(), and ignore all allocator-internal
frames.  Prior to this change, pprof excluded the specifically
blacklisted functions from backtraces, but it left allocator-internal
frames intact.