[runtimes] Reintroduce a way to select the compiler used for the test suite (#93542)

A while back, the cxx_under_test Lit parameter was removed. This patch
reintroduces a Lit parameter called "compiler" which controls the value
of the %{cxx} substitution used in the test suite.

To run the test suite with a different compiler, one can now pass
--param compiler=<path>.