Rework WITHOUT_LLD/TOOLCHAIN fix from r327892 for cross-tools.

MK_LLD is for the installed lld while MK_LLD_BOOTSTRAP is for the build
tool. For WITH_SYSTEM_LINKER it is necesarry to separate the logic of
these two. When building libllvm TOOLS_PREFIX will be defined and
MK_LLD_BOOTSTRAP should be checked instead.

Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D15837