[compiler-rt] [compiler-rt] Add CMake option to enable execute-only code generation on AArch64 (PR #140555)

[Csanád Hajdú via llvm-commits]

Il-Capitano wrote:

> I don't think that this properly handles MachO.

Can you expand on this more? MachO doesn't have execute-only code AFAIK. This is intended for AArch64 ELF targets only.

> Additionally, I don't understand the need for the `unique` flag on the section, nor the section id (`0`).

`unique,0` is needed because the assembler creates an implicit `.text` section with default flags. If we tried to just do `.section .text,"axy"`, we'd get the following error: `error: changed section flags for .text, expected: 0x6`. So the execute-only `.text` section needs to be different from the implicit `.text` section. The id `0` is arbitrary, but we need to provide something anyways.

This also matches what LLVM outputs when it generates execute-only code: https://godbolt.org/z/a8xa7qsxG

https://github.com/llvm/llvm-project/pull/140555