[ASan] Deduplicate interception-in-shared-lib-test.cc by introducing...

[ASan] Deduplicate interception-in-shared-lib-test.cc by introducing platform-specific substitutions for rpath linker flags

Also make suppressions-library.cc use the same flags to avoid warnings about unused -rpath flags.
The same substitutions will be used to make coverage tests work on both Linux and Darwin without duplicating the code.

llvm-svn: 233802