debugserver: Rely on mig architectures being specified externally

Look up the -arch flags to pass to the mig invocation from an
optionally-defined MIG_ARCHS variable. We can't use CMAKE_OSX_ARCHS
because the {i,tv,watch}OS builds don't use this mechanism to achieve
fat builds (they build each slice separately & then lipo them together).

This supercedes the mig -arch/-isysroot fix from
510758da.