Use .eh_frame/.debug_frame unwind information to improve callstack walking

DrMemory's callstack walker uses solely stack scanning and frame pointers, but it has some shortcomings. We have assumed that going into the DWARF .eh_frame/.debug_frame section would be too expensive, but given recent problems with skipping frames in simple apps due to libc optimizations requiring expensive stack scans in any case, it is worth implementing and having available and evaluating performance for whether to have it on all the time.

The corresponding Windows feature is #1222.