Update wasi-libc to sdk-19, with patch to null out bulk memory opcodes that the mozilla-{release,esr} codebase fails to build with.
wasi-libcxx and wasi-compiler-rt now meta ports; the build process is the same for each LLVM version so child ports are minimal. Flavours don't work here due to differing LLVM versions between them. Note that LLVM 16 changed the directory hierarchy to major version only, so cannot use exact file references for the wasi-compiler-rt dependency line any longer.
Since all supported LLVM versions are made available at once, remove restriction in bsd.gecko.mk. While here switch to USES=llvm throughout, max version limited to 15 for now (see below).