[ASan] Add new options for asan_symbolize.py script.

The patch adds new features in asan-symbolizer script which are helpful for using ASan on embedded systems:

1) add cross-compile prefix for binutils
2) define path to sysroot with sanitized binaries

Features are enabled by command line options.
The patch also extends command line interface with help option.

Reviewed in http://reviews.llvm.org/D4703.

Patch by Maria Guseva!