add `-Zmin-function-alignment`
tracking issue: https://github.com/rust-lang/rust/issues/82232
This PR adds the `-Zmin-function-alignment=<align>` flag, that specifies a minimum alignment for all* functions.
### Motivation
This feature is requested by RfL [here](https://github.com/rust-lang/rust/issues/128830):
> i.e. the equivalents of `-fmin-function-alignment` ([GCC](https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fmin-function-alignment_003dn), Clang does not support it) / `-falign-functions` ([GCC](https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-falign-functions), [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang1-falign-functions)).
>
> For the Linux kernel, the behavior wanted is that of GCC's `-fmin-function-alignment` and Clang's `-falign-functions`, i.e. align all functions, including cold functions.
>
> There is [`feature(fn_align)`](https://github.com/rust-lang/rust/issues/82232), but we need to do it globally.
### Behavior
The `fn_align` feature does not have an RFC. It was decided at the time that it would not be necessary, but maybe we feel differently about that now? In any case, here are the semantics of this flag:
- `-Zmin-function-alignment=<align>` specifies the minimum alignment of all* functions
- the `#[repr(align(<align>))]` attribute can be used to override the function alignment on a per-function basis: when `-Zmin-function-alignment` is specified, the attribute's value is only used when it is higher than the value passed to `-Zmin-function-alignment`.
- the target may decide to use a higher value (e.g. on x86_64 the minimum that LLVM generates is 16)
- The highest supported alignment in rust is `2^29`: I checked a bunch of targets, and they all emit the `.p2align 29` directive for targets that align functions at all (some GPU stuff does not have function alignment).
*: Only with `build-std` would the minimum alignment also be applied to `std` functions.
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cc `@ojeda`
r? `@workingjubilee` you were active on the tracking issue
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