Log Message:
wasi-compiler-rt & wasi-libcxx: build fix for firefox

BUILDLINK_API_DEPENDS must reflect the exact llvm version, as this is
necessary to force the wasi packages to match, or build failures will
occur, because various commands invoked in the Firefox build process
supply the exact clang version in their path. For instance:

wasm-ld: error: cannot open \ 
/usr/pkg/lib/clang/13.0.1/lib/wasi/libclang_rt.builtins-wasm32.a: No such file \ 
or directory
clang-13: error: linker command failed with exit code 1 (use -v to see invocation)

(No PKGREVISION bump of www/firefox required, as this is a build-only
dependency, and no other packages presently depend on these.)