[ulib][backtrace] Fix leaks of strtab,debug views.

Still needs pecoff equivalent, but this is enough for magenta.

Change-Id: If038566cf9dcd3019b30244752ec5a06185d77dd

[ulib][backtrace] Add function backtrace_set_base_address.

This is useful when just using libbacktrace to get
file,line#,function_name, and the caller is doing its own unwinding.

Change-Id: I39bdb56fe74f4801705836c972b48167507ce6aa

[ulib][backtrace] New function backtrace_set_so_iterator.

This function lets one provide a different iterator than
dl_iterate_phdr for ELF (which is still the default).

Change-Id: I6e6da79364aab104ccea082ccfdb8a305443f1c1

[ulib][backtrace] Initial import.

git clone https://github.com/ianlancetaylor/libbacktrace.git
upstream commit: ba79a27ee9a62b1be86d0ddae7614c316b7f6fbb

Change-Id: I3367a96bbf6b42141c2440ce6568a5cc5375d8ee