Using the glibc compatibility requires librt.so, but it is not part of
the depends subsystem.
This diff adds the cross compiler installation dir to the search path:
- By setting the search prefix explicitely for ARM;
- By using the system path as a fallback to support the x86 platform.
Because of CMake search priority, the libraries from the depends subsystem will always be found first (if they exist).
Depends on D4038.