arch/xap: refactor the code up to rizin standard

[wargio]

SQUASH ME

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

Updates xap arch to use the latest api.

• Fixes output using RzStrBuf directly.
  (Before)
  
  (After)
  
• Removes useless global variable.

[XVilka]

After this one is merged, we also should update Rizin submodule in Cutter, there are quite a few fixes since the last time we did it. Initially I wanted to wait until MIPS, nanoMIPS, and Xtensa lands, but that could be done again after.