Requiring Specific Universal_Archs

[Jeremy Lavergne]

A port I'm working on has this requirement:
 * +universal may only be {i386 ppc}
 * +universal may not be used for specific OS versions

I can use `array size ...` and two `lsearch ...` to ensure only i386 and
ppc are in the universal_archs, and can test the os.version when
+universal is active. Is that the preferred means of following the port's
requirements?