[llvm][cmake] Shore up `llvm_check_linker_flags`.

Ensure `CMAKE_EXE_LINKER_FLAGS` effects the underlying `try_compile` by setting
this policy to use the new, non-deprecated behavior, without effecting the
caller.

Normally this shouldn't be necessary, as CMake uses
`cmake_minimum_required(VERSION 3.13.4)` consistently, but I suppose
this could rear its head in a downstream project?

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D118546