Add LLVM_TARGET_ALL option.

LLVM_TARGET_* will auto be set based on LLVM_TARGET_ALL and MK_CLANG.

If LLVM_TARGET_ALL is disabled, during a cross-build, then SYSTEM_COMPILER
and SYSTEM_LINKER are auto disabled.

This option should be used by users rather than the per-arch LLVM_TARGET
options as it is simpler to maintain for them should the supported
target list change.

MFC after: 2 weeks
Reviewed by: sbruno, dim
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D16020