Fix the addition of Clang's profile runtime library to the link step
when specifying --coverage (or related) flags.

The system for doing this was based on the old LLVM-hosted profile_rt
library, and hadn't been updated for Linux to use the new compiler-rt
library. Also, it couldn't possibly work on multiarch or biarch systems
in many cases. The whole thing now works much the same as the sanitizer
libraries that are built and used out of the compiler-rt repo.

Note that other target OSes haven't been updated because I don't know if
they're doing anything special with the installation path of profile_rt.
I suspect however that *all* of these are wrong and would encourage
maintainers of each target to take a hard look at how compiler-rt
runtime libraries are linked on their platforms.