[Compiler-rt] Downgrade fatal error about unsupported test configuration

to a warning.

https://reviews.llvm.org/D101681 introduced a check to make sure the
compiler and compiler-rt were using the same library path when
`COMPILER_RT_TEST_STANDALONE_BUILD_LIBS=ON`, i.e. the developer's
intention is to test the just built libs rather that shipped with the
compiler used for testing.

It seems this broken some bots that are likely misconfigured.

So to unbreak them, for now let's make this a warning so the bot
owners can investigate without breaking their builds.