Remove dependency on gcc/clang

[roblabla]

Right now, we use a C compiler to compile svc.S. This is completely stupid. Let's fix that.

I can see at least two ways to fix that:

• Simply pull the svc.S into a global_asm! block in svc.rs. This is the easiest way.
• Use naked functions and inline asm to recode them. This gives us additional flexibility if we want to make the API more rusty.

Worth noting that the current svcs are basically a copy paste from libt, and are very C-like. I would love to turn them into something closer to rust, and would probably be possible without too much trouble.

[roblabla]

Fixed