wasi: update libc to sdk-20 and modularise LLVM bits

wasi-libcxx and wasi-compiler-rt now meta ports (similar to u-boot);
the build process is the same for each LLVM version so child ports
are minimal. Minimum supported LLVM version is 12, thus its
corresponding child ports are resurrected and LLVM 11 child ports
are removed.

Since all supported LLVM versions are made available at once, remove
restriction in bsd.gecko.mk and switch to USES=llvm throughout. LTO
on gecko ports should now work with later LLVM versions so long as
${DEFAULT_VERSIONS} is set properly and matches the LLVM used in
rust (if not using the PORT_LLVM option).

Reviewed by: brooks (LLVM ports interactions)
Tested by: Eric Camachat <eric[at]camachat[dot]org>
Approved by: maintainer timeout (gecko@ portion, 2+ weeks)
PR: 274896
Differential Revision: https://reviews.freebsd.org/D40098