[Driver] Fix handling of -fbuiltin/-fcommon when combined with -mkernel
-mkernel enables -fno-builtin and -fno-common by default, but allows -fbuiltin
and -fcommon to override that. However "-fbuiltin -fno-builtin" is treated the
same as "-fbuiltin" which is wrong, so fix that. Also fixes similar behaviour
when -fno-common is default.
Differential Revision: http://reviews.llvm.org/D11459