[clang] [llvm] [LLVM][Clang][AArch64] Implement AArch64 build attributes (PR #118771)

[Oliver Stannard via llvm-commits]

ostannard wrote:

> Not added: Tests for attributes with NTBS values - currently all attributes takes only ULEB128 as values. In order to have attributes that takes NTBS values, new headers need to be defined.

The `.aeabi_subsection` directive should accept any subsection name, only checking the second and third argument if the name is known. This will allow adding new, possibly platform-specific subsections without needing to update the assembler to know about them, and is why the `.aeabi_subsection` directive has the `optional` and `type` arguments. This will then make it possible to emit attributes with NTBS values.

https://github.com/llvm/llvm-project/pull/118771