[CMake] Fix runtimes cross-compilation when using LLVM_USE_LINKER

[thevinster]

Summary

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.

[phosek]

LGTM

[smeenai]

LGTM