[flang-rt] Make `FLANG_RT_INSTALL_RESOURCE_PATH` configurable (#128561)
Make it possible to change the path used to install flang-rt library.
This is particularly necessary for standalone builds, where the CMake
script currently hardwires the default clang install path, and therefore
is incorrect for distributions that override it. However, for
consistency I have made it configurable unconditionally, preserving the
current defaults.