Add Zfh into ELF march attribute.
Having each X and Z extension hard-code its length is going to get ugly (and this doesn't match the code style for variable names). Instead I think we should be making good use of StringRef::consume_front.
This changes the test; we're no longer checking whether D implies F but instead whether (D or Zfh) implies F.
For now, startswith would work due to the drop_until below. However, consume_front matches the take_front above, and will be important when we want to start parsing version numbers.
... ok but now we drop that many characters twice: once from consume_front and once from drop_front below.