[llvm-size] Reject unknown radix values

This addresses https://bugs.llvm.org/show_bug.cgi?id=39403 by making
-radix an enumeration option with 8, 10, and 16 as the only accepted
values.

Reviewed by: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D53799

Patch by Eugene Sharygin

llvm-svn: 345588