[llvm] Forward install RPATH setting to runtimes (PR #129496)

[Petr Hosek via llvm-commits]

petrhosek wrote:

This would use the same `RPATH` for all targets and I'm not sure if that's desirable.

To set `RPATH` for individual targets, you can use:
```
set(RUNTIMES_${TARGET_TRIPLE}_CMAKE_INSTALL_RPATH "${INSTALL_RPATH}" CACHE STRING "")
set(RUNTIMES_${TARGET_TRIPLE}_CMAKE_BUILD_WITH_INSTALL_RPATH ON CACHE BOOL "")
```

https://github.com/llvm/llvm-project/pull/129496