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