Add "gas", "gnu asm", and "unix asm" aliases to Unix Assembly (#5923)
Add "gas", "gnu asm", and "unix asm" aliases to Unix Assembly (#5923)

* Add 2 new aliases to Unix Assembly

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

* Apply suggestions from code review

Co-authored-by: John Gardner <e36d49b36f1c453256531efcafb5a1ecb0806815@gmail.com>

Co-authored-by: John Gardner <e36d49b36f1c453256531efcafb5a1ecb0806815@gmail.com>
github/linguist,github/linguist,github/linguist,github/linguist,erkyrath/linguist,erkyrath/linguist,erkyrath/linguist,erkyrath/linguist