www/firefox: make dep on wasi-compiler-rt more flexible

as firefox needs matching wasi-compiler-rt, embedded llvm in
rust and llvm for building and wasi-compiler-rt installs it's files
in a path depending on it's full distversion, we need to know
llvm/clang's full version. it's not possible to derive that
programatically, as the required llvm version may not be installed
at the time firefox' Makefile is being parsed.