Fixes rust-lang#64412

Following up on [1] and [2], this PR adds differntiation for aarch64 bare-metal
targets between versions with and without hardware floating point enabled.

This streamlines the target naming with other existing ARM targets and provides
the user clear indication if he is getting float or non-float for his bare-metal
target.

[1] rust-lang#60135 (comment)
[2] rust-embedded/wg#230

Closes: rust-embedded/wg#230

…standard library

This can cause cycles as `ConstProp` uses `layout_of` which, for
generators, uses `optimized_mir` which runs `ConstProp`.

This allows us to avoid changing things directly in the miri engine just
for const prop.

…li-obk

Deduplicate some code between miri and const prop

r? @oli-obk

…on-E0531, r=Centril

Add Long error explanation for E0531

Part of rust-lang#61137.

…e-tests, r=nikomatsakis

Bugfix/rfc 2451 rerebalance tests

r? @nikomatsakis

Fixes rust-lang#64412
Depends/Contains on rust-lang#64414

cc rust-lang#55437 and rust-lang#63599

…=Amanieu

Differentiate AArch64 bare-metal targets between hf and non-hf.

CC @parched, kindly request you to review.

~~Note: This change breaks code that uses the target `aarch64-unknown-none` for the sake of clearer naming as discussed in the links posted below. A search on github reveals that code using `aarch64-unknown-none` is almost exclusively forked from our embedded WG's OS tutorials repo at https://github.com/rust-embedded/rust-raspi3-OS-tutorials, for which the target was originally created.~~

~~I will adapt this repo to the new target name asap once this PR would go upstream. The minor annoyance for the forks to break temporarily should be acceptable for the sake of introducing a better differentiation now before it is too late.
Also, the break would only happen upon updating the toolchain, giving the user a good hint at what has happened.~~

---------- Patch commit message:
Following up on [1] and [2], this PR adds differntiation for aarch64 bare-metal
targets between versions with and without floating point enabled.

This streamlines the target naming with other existing ARM targets and provides
the user clear indication if he is getting float or non-float for his bare-metal
target.

[1] rust-lang#60135 (comment)
[2] rust-embedded/wg#230

Closes: rust-embedded/wg#230

…-E0734, r=estebank

Add E0734 and its long explanation

Part of rust-lang#61137