[Runtimes] Introduce variables containing resource dir paths (#177953)

Introduce common infrastructure for runtimes that determines compiler
resource path locations. These variables introduced are:

 * RUNTIMES_OUTPUT_RESOURCE_DIR
 * RUNTIMES_INSTALL_RESOURCE_PATH

That contain the location for the compiler resource path (typically
`lib/clang/<version>`) in the build tree and the install tree (the
latter relative to CMAKE_INSTALL_PREFIX).

Additionally, define

 * RUNTIMES_OUTPUT_RESOURCE_LIB_DIR
 * RUNTIMES_INSTALL_RESOURCE_LIB_PATH

as for the location of clang/flang version-locked libraries (typically
`lib${LLVM_LIBDIR_SUFFIX}/<targer-triple>`, but also depends on `APPLE`
and `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR`). This code is moved from
flang-rt and initially becomes its only user.

Refactored out of #171610 as requested
[here](https://github.com/llvm/llvm-project/pull/171610#discussion_r2687382481).

Extracted `get_runtimes_target_libdir_common` from compiler-rt as
requested
[here](https://github.com/llvm/llvm-project/pull/171610#discussion_r2689565634).

Added TODO comments to all runtimes as requested
[here](https://github.com/llvm/llvm-project/pull/171610#issuecomment-3789598635).

GitOrigin-RevId: afb80bddf147c134c5e3cf3f357f8d00bdfd526c