testsuite: Make ghc_built_by_llvm check more precise

Previously it would hackily look at the flavour name to determine
whether LLVM was used to build stage2 ghc. However, this didn't work at
all with Hadrian and would miss cases like ARM where we use the LLVM
backend by default.

See #16087 for the motivation for why ghc_built_by_llvm is needed at
all. This should catch one of the ARMv7 failures described in #17555.