The PR is to generalize the re-use of the `compilerRT` code of adding
the path of `libflang_rt.runtime.a (so)` from AIX and LoP only to all
platforms via a new function `addFlangRTLibPath`.
It also added `-static-libflangrt` and `-shared-libflangrt` compiler
options to allow users choosing which `flang-rt` to link to. It defaults
to shared `flang-rt`, which is consistent with the linker behavior,
except on AIX, it defaults to static.
Also, PR #134320 exposed an issue in PR #131041 that the the overriding
`addFortranRuntimeLibs` is missing the link to `libquadmath`. This PR
also fixed that and restored the test case that PR #131041 broke.
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