Revert r335449 and add needed MK_LLD_BOOTSTRAP check for SRCS_MIW.

This effectively reverts r335449 and changes the previous MK_LLD_IS_LD
to a MK_LLD_BOOTSTRAP check. If !TOOLS_PREFIX then these sources are
always built for llvm-objdump, lld, and llvm-cov. When TOOLS_PREFIX
is set then they are only needed if lld is being bootstrapped.

Reported by: dim
Pointyhat to: bdrewery
Sponsored by: Dell EMC