[CMake] Fix runtimes cross-compilation when using LLVM_USE_LINKER

[CMake] Fix runtimes cross-compilation when using LLVM_USE_LINKER

This is motivated by https://reviews.llvm.org/D115852#3226050 where passing
`LLVM_USE_LINKER` in PASSTHROUGH_PREFIXES breaks cross-compilation
(e.g. targeting Windows on a Linux host). As suggested on that diff, implement
an override for it by allowing users to pass `RUNTIMES_${target}_LLVM_USE_LINKER`
to specify the linker for runtimes. Defaults to host linker if not specified.

Reviewed By: phosek, smeenai

Differential Revision: https://reviews.llvm.org/D131051