Add 2 new aliases to Unix Assembly

[Frityet]

Currently, it seems to be impossible (in my testing) to override a file type to be Unix Assembler. This commit should fix that. Unix Assembler uses the AT&T syntax of x86 assembly, and so the first alias is at&tasm. The second alias, unixasm is just a shortened version of the name for linguist

Description

Checklist:

• I am associating a language with a new file extension.

  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3AFOOBAR+KEYWORDS+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am adding a new language.

  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3AFOOBAR+KEYWORDS+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a syntax highlighting grammar: [URL to grammar repo]
  • I have updated the heuristics to distinguish my language from others using the same extension.

• I am fixing a misclassified language

  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am changing the source of a syntax highlighting grammar

  • Old: [URL to grammar repo]
  • New: [URL to grammar repo]

• I am updating a grammar submodule

• I am adding new or changing current functionality

  • I have added or updated the tests for the new or changed functionality.

• I am changing the color associated with a language

  • I have obtained agreement from the wider language community on this color change.
    • [URL to public discussion]
    • [Optional: URL to official branding guidelines for the language]

[Alhadis]

Strange, I could've sworn that gas was supported as an alias. Perhaps we could add that instead?

Currently, it seems to be impossible (in my testing) to override a file type to be Unix Assembler.

You can use unix-assembly:

```unix-assembly
# Comment
_start:
```

Output:

# Comment
_start:

[Frityet]

the GNU assembler (GAS) supports more than just AT&T syntax thouhg

[Alhadis]

the GNU assembler (GAS) supports more than just AT&T syntax thouhg

Via .intel_syntax, you mean? Yes, that's true. But I'm pretty sure folks won't get confused; the default assembly highlighting seems to be pretty Intel-centric, so it's not a stretch to assume that folks with associate GNU assembler with AT&T syntax (or perhaps I'm just being optimistic).

[Frityet]

the GNU assembler (GAS) supports more than just AT&T syntax thouhg

Via .intel_syntax, you mean? Yes, that's true. But I'm pretty sure folks won't get confused; the default assembly highlighting seems to be pretty Intel-centric, so it's not a stretch to assume that folks with associate GNU assembler with AT&T syntax (or perhaps I'm just being optimistic).

I actually mainly meant arm assembly (the reason why i made this pr lol)