cmake: improve CMAKE_SYSTEM_PROCESSOR assignment in nativesdk

cmake: improve CMAKE_SYSTEM_PROCESSOR assignment in nativesdk

The previous string manipulations would result in the wrong string being used
for machines such as intel-corei7-64 as the sysroot was split at the first
hyphen (so would result in corei7 instead of corei7-64).

Change the logic so that it looks for processor-distro-os and uses the whole of
the processor field.

Signed-off-by: Ross Burton <ross.burton@intel.com>