WebAssembly: add a generic CPU

Summary: WebAssemblySubtarget.cpp expects a default 'generic' CPU to exist, and this seems to be prevalent with other targets. It makes sense to have something between MVP and bleeding-edge, even though for now it's the same as MVP. This removes a warning that's currently generated.

Subscribers: jfb, llvm-commits, sunfish

Differential Revision: http://reviews.llvm.org/D11546

llvm-svn: 243345