arm64 is the only currently supported architecture that has ${MACHINE_CPUARCH} set to a different value (aarch64) than ${MACHINE} (arm64), as described in arch(7). However, there is no source directory associated with arm64 that has a name set to ${MACHINE_CPUARCH}.
Remove the dead code that adds a directory with a name set to ${MACHINE_CPUARCH} to a list of directories indexed with cscope. This change allows to use the cscope target on arm64.
My branch:
https://github.com/kwitaszczyk/freebsd/tree/cscope-arm64