Clang: Only build needed target for bootstrap compiler.

This will disable the new LLVM_TARGET_ALL option which will only
enable the required target.

This only impacts the bootstrap compiler in WORLDTMP, not the target compiler
that will be installed.

MFC after: 2 weeks
Reviewed by: sbruno, dim (earlier version)
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D16021