llvm update to 20.1.8

Hi!

I've worked on an update for llvm & all related packages to 20.1.8, as
well as some other packages that needed build fixes for the new llvm
version. The update is based on adam's work up to commit
1f701ceffc8429f61d994f9cb829b4c07e4f4bf4 in wip (wip is already
tracking llvm 21) with additional fixes.

This llvm update allow upgrading zig (included) and rust (not
included) to newer versions.

I've tested bulk-test-llvm (including rust, zig, firefox and
thunderbird) on NetBSD 11.99.5/x86_64, and it built except for two
packages:

mlir:

fails, not finding libMLIR.* - reported upstream at
https://github.com/llvm/llvm-project/issues/181584

lldb:

fails in the NetBSD specific code - reported upstream at
https://github.com/llvm/llvm-project/issues/181586

lldb is a leaf packages (except for the bulk test package), mlir is
used by flang.

I'd like to go ahead with the update anyway so we can get newer rust &
zig.

The update is here[1]:

https://www.netbsd.org/~wiz/llvm-20.5.diff

Please review & test, and if you have ideas for fixes for the two
problems listed above, I'd appreciate it.

Thanks,
 Thomas


[1] The update is not in wip because wip is not well suited for this
use case (you could install the packages from there, but the
dependencies in the standard pkgsrc tree would have to be manually
changed to point to wip for bulk builds to succeed) - pkgsrc branches
would be better.