[compiler-rt][CMake] Properly set COMPILER_RT_HAS_LLD

LLVM_TOOL_LLD_BUILD is a relic of the pre-monorepo times. This causes us to never set COMPILER_RT_HAS_LLD.

Instead, set it from the runtimes build if lld is being built and lld is used as the compiler-rt linker.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D144660