wasi: drop support for LLVM < 17

Prepare for update

devel/wasi-libc: update to sdk-21

Bump PORTREVISION of consumers

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

Revert "devel/wasi-compiler-rt14: add new port"

Charlie Li <vishwin@freebsd.org> requests a revert of the addition of
this port, since it needs updates to wase-libc and wasi-libc++ to be
used.

The support for LLVM-14 has been worked on since May of 2022, with a
number of unresolved issues remaining (see reviews D35286 to D35389).

The "make index" breakage for LLVM_DEFAULT=14 has been dealt with by
enforcing use of LLVM-13 for www/firefox (in commit 6b2cf7d799d).

This reverts commit febcdcce6160c7203722afca2c226aeac531c1e6.

devel/wasi-compiler-rt14: add new port

This port is a copy of devel/wasi-compiler-rt13 with the DISTVERSION
changed to 14.0.6 and the distinfo file updated to match that version.