Avoid -lgcc for heap profiling if unwind.h is missing.
Avoid -lgcc for heap profiling if unwind.h is missing.

This removes an unneeded library dependency when falling back to
intrinsics-based backtracing (or failing to enable heap profiling at
all).

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc