cmd/vet: move asm checks into cmd/asm

Currently cmd/vet does not expand preprocessor macros in assembly code. As a result, it missed several issues in the runtime (a73d68e) and x/crypto (golang/crypto@5953a47, golang/crypto@e67f5ec, golang/crypto@3c0d69f). It would be nice if it could preprocess the assembly code before vetting it.
