Set CMAKE_SYSTEM_PROCESSOR when cross-compiling
Set CMAKE_SYSTEM_PROCESSOR when cross-compiling

Though rare, some packages have cpu-specific branches in their
CMakeLists which might fail if the CMAKE_SYSTEM_PROCESSOR is empty which
is the default when cross-compiling.

With this change the CMAKE_SYSTEM_PROCESSOR will be set in the toolchain
file from pkg_build_cpu (build.cpu) if it is set or from pkg_build_arch
(build.arch) as a fallback which itself is determined from the first
component of the build.system property. This property is inherited from
the toolchain, so no manual changes to the rules should be necessary.

bazurbat/jagen