Use specific clang assembler when compiling with -fllvm
There are situations where LLVM will produce assembly which older gcc toolchains can't handle. For example on Deb10, it seems that LLVM >= 13 produces assembly which the default gcc doesn't support.
A more robust solution in the long term is to require a specific LLVM compatible assembler when using -fllvm.
Fixes #16354 (closed)