[ARM] Fix crash "-target arm -mcpu=generic", without "-march="

An assertion hit has been fixed for cmdlines like

$ clang --target=arm-linux-gnueabi -mcpu=generic hello.c

Related to: http://reviews.llvm.org/rL245445

Reviewers: rengolin

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13013

llvm-svn: 248370