Support for assembling slti instructions.

[matteius]

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

I extended my local version of cutter to be able to insert multiple MIPs instructions at once, then I realized it can dissasemble slti isntructions but not re-assemble them so I was getting errors inserting these.

Test plan

Just trying to help, but before this change this would fail:

(base) matteius@matteius-3-0:~/cutter/build$ ./Rizin-prefix/bin/rz-asm -a mips -b 32 'slti v0, a0, 9'
09008228

Closing issues
Not sure if this has been reported in any issues yet.

[wargio]

please add a test under the tests/db/asm dir

[matteius]

@wargio Are the asm tests there just testing that it can compute the assembly of an instruction using in this case slti? Also I don't see a regular mips32 or mips test file, but I do see mips64 ...

[wargio]

you are right. that is indeed an issue. ok for me then.

[wargio]

#4509