Change register used in setting SS in stage_4

[Elekrisk]

The register bx is apparently reserved by LLVM, and building with the latest nightly (1.54-nightly) fails because of it:

error: invalid register `bx`: rbx is used internally by LLVM and cannot be used as an operand for inline asm
  --> src\bin\bios.rs:55:9
   |
55 |         out("bx") _,
   |  

This pull request changes the register used to ax.
I don't think this'll be able to break anything. cargo test passes, and building and running my own kernel works.

[phil-opp]

Thanks for the pull request! I just restarted our CI job in order to test it with the latest nightly and I can confirm that the mentioned error occurs. It looks like it is caused by rust-lang/rust#84658. So apparently using rbx was already incorrect before, it just didn't lead to an error.

[phil-opp]

Published as v0.10.3.