Addresses #3809: when we are asked to create a Cranelift backend with
shared flags that indicate support for SIMD, we should check that the
ISA level needed for our SIMD lowerings is present.
`ptr::cast` has the advantage of being unable to silently cast
`*const T` to `*mut T`. This turned up several places that were
performing such casts, which this PR also fixes.
This also paves the way for unifying TargetIsa and MachBackend, since now they map one to one. In theory the two traits could be merged, which would be nice to limit the number of total concepts. Also they have quite different responsibilities, so it might be fine to keep them separate.
Interestingly, this PR started as removing RegInfo from the TargetIsa trait since the adapter returned a dummy value there. From the fallout, noticed that all Display implementations didn't needed an ISA anymore (since these were only used to render ISA specific registers). Also the whole family of RegInfo / ValueLoc / RegUnit was exclusively used for the old backend, and these could be removed. Notably, some IR instructions needed to be removed, because they were using RegUnit too: this was the oddball of regfill / regmove / regspill / copy_special, which were IR instructions inserted by the old regalloc. Fare thee well!
* Introduce new_got_entry and new_plt_entry functions
* Return NonNull<*const u8> from get_got_address
* Make GOT entry writes atomic
* Defer GOT updates until relocations and protection
Co-authored-by: Alan Egerton <eggyal@gmail.com>
