[cmake] Pass -resource-dir to external projects as CMAKE_<LANG>_FLAGS_INIT...

[cmake] Pass -resource-dir to external projects as CMAKE_<LANG>_FLAGS_INIT instead of as a force override (#87330)

It's useful to use toolchain files for configuring runtimes builds.
Setting `CMAKE_<LANG>_FLAGS` is however forceful and precludes initialization from the _INIT variant meant to be set by toolchains.
The toolchain file still needs to append instead of just setting and be idempotent (`include_guard()`), but this way at least a special toolchain to account for LLVM won't be required.