[Sanitizer] Add rudimentary support for using libbacktrace in symbolizer.

More steps are needed to actually make it usable:
* sanitizer runtimes should be compiled with -DSANITIZER_LIBBACKTRACE.
* libbacktrace headers should be installed.
* user has to manually link in libbacktrace.a into the executable.

We can easily solve the first two problems in the build system, but
detecting/linking libbacktrace to all the tests we have and end-user programs
is more challenging (and will unlikely work w/o Driver support).

Based on the patch by Jakub Jelinek!


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@195837 91177308-0d34-0410-b5e6-96231b3b80d8