[asan] Add %d variable to external_symbolizer_path option, so that user can...

[asan] Add %d variable to external_symbolizer_path option, so that user can specify paths relative to the location of the binary.

We want way to set a path to llvm-symbolizer that isn't relative
to the current working directory; this change adds a variable that
expands to the path relative to the current binary.
This approach came from comments in https://reviews.llvm.org/D93070

Differential Revision: https://reviews.llvm.org/D94563