Add a builder to sv::VerbatimOp that accepts a Twine argument. This will make it a lot easier to construct strings that need to be passed in without having to do an explicit conversion to a string beforehand.

This was suggested by @youngar in https://github.com/llvm/circt/pull/1170/files#r649805728 (h/t).

@honglh added optimized kernels for ARM32 in #432. It would be great if we could add support for bitpacked activations to them to match the AArch64 optimized bgemm kernels.