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

This change allows to compile sanitizer sources so that *san runtime
will attempt to use libbacktrace and/or libiberty for symbolization
(instead of communicating with llvm-symbolizer).

I've tested this patch by manually defining SANITIZER_LIBBACKTRACE and/or
SANITIZER_CP_DEMANGLE, linking with necessary libraries and verifying that
all tests from ASan test suite work.

Based on patches by Jakub Jelinek!

llvm-svn: 199384