Handle arm/armv[67] hosted/targeted builds gracefully

CPUTYPE (apparently) isn't defined in non-cross-builds, which caused
arm/armv[67] hosted/targeted builds to fail when evaluating CPUTYPE.

Add the :U modifier to CPUTYPE so it evaluates to "". This allows armv[67] to
get past the conditional successfully.

MFC after: 1 week
Reported by: bob prohaska <fbsd@www.zefox.net>