[clang] [ARM] Aligned mtp behavior and gcc (PR #128728)

[Simon Tatham via cfe-commits]

https://github.com/statham-arm commented:

Would it be possible to modify the description of this PR so that it generates a commit message that explains the change on its own, without having to refer to a Github ticket containing a discussion?

If I were writing this change, I'd write a commit message something like the following. (You're welcome to use this template if you like, although the [FIXME]s should be edited to fill in the actual facts.)

>[ARM] Introduce -mtp=auto and make it the default
>
>This adds a new value `auto` to the possible values of the existing `-mtp=` clang option which controls how the thread pointer is found. `auto` means the same as `soft` if the target architecture doesn't support a hardware thread pointer at all; otherwise it means the same as [FIXME].
>
>This behavior is the default in gcc version [FIXME] and later. The new `auto` option is therefore also the default in clang, so this change aligns clang with gcc.

https://github.com/llvm/llvm-project/pull/128728