Use GNU as pseudo-ops

[Yamakaky]

https://sourceware.org/binutils/docs/as/Pseudo-Ops.html
Instead of inventing an other syntax, why not using a popular one? + It would be easier to document: just say which ops are supported!

[Yamakaky]

Bonus: it would be easier to port GNU as, clang as... to the DCPU.
I'm also in favour to support C-style preprocessor.