[cmake] Allow overriding Clang resource symlink target (#41301)
When building with a prebuilt Clang, the changes introduced in #40707
supposed that the toolchain was in its final path.
When building in stages (first the toolchain, then the standard
library), the toolchain might not be in the final path, and the created
symlink will point to a machine-local path that does not make sense.
The changes introduced should not modify the existing behaviour
introduced by #40707, but should allow customizing the final
installation target using the CMake cached variable for those setups
that need the flexibility.
