1bc2446c78
Other tools, such as `llc`, use `generic` cpu by default, if you don't
give any `-mcpu`:
75f738b0b2/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.cpp (L38-L39)
But `llvm-mc` didn't do that. This makes `generic` also the default CPU
for `llvm-mc`.