GAS should be a language, not compiled with any GNU compiler

[dcbaker]

This leads to a couple of problems by not treating it as a language:

• you must have a GNU compiler to be able to compile it, even though you could use gas as a standalone implementation, even we need to use cpp or a C compatible compiler to do the pre-processing of .S files
• it means that we have to put arguments for assembly in with C, C++, ObjC, or Fortran arguments, and hope that they are always added in the right place. Some of these arguments are only valid for GAS assembly or for the higher level language, which means we can't avoid warnings/errors (BUG?: Passing flags for `.S` doesn't seem to work #13374)
• It means that we don't do up front validation that we actually have the necessary support for GAS like we do with other assemblers and compilers

Complications:

• We need a backwards compatibility story, since lots of projects likely rely on the current behavior, and breaking them isn't great.