Fix generic asm color inconsistencies

[giridharprasath]

• Enhanced unknown string parsing in asm generic tokenization

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

• Added more checks for parsing the generic asm str buf

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Test plan
Tested on 2 binaries (x86-64, arm64)
Output before:

Now:

Before:
ch was getting recognized as a register

Now:

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Closing issues

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