libstd: -Z build-std is now broken for Cortex-M targets #98378

$ rustup default nightly-2022-06-22
$ rustup component add rust-src
$ rustup component add thumbv7m-none-eabi

$ cargo new --lib foo
$ cd foo
$ echo '#![no_std]' > src/lib.rs

$ cargo build -Z build-std --target thumbv7m-none-eabi
(..)
error: `sys_common::condvar::Condvar::new` is not yet stable as a const fn
   --> /home/japaric/.rustup/toolchains/nightly-2022-06-22-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sync/condvar.rs:129:26
    |
129 |         Condvar { inner: sys::Condvar::new() }
    |                          ^^^^^^^^^^^^^^^^^^^
    |
    = help: const-stable functions can only call other const-stable functions

error: `sys_common::mutex::MovableMutex::new` is not yet stable as a const fn
   --> /home/japaric/.rustup/toolchains/nightly-2022-06-22-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sync/mutex.rs:220:20
    |
220 |             inner: sys::MovableMutex::new(),
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: const-stable functions can only call other const-stable functions

error: `sys_common::rwlock::MovableRwLock::new` is not yet stable as a const fn
   --> /home/japaric/.rustup/toolchains/nightly-2022-06-22-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sync/rwlock.rs:153:20
    |
153 |             inner: sys::MovableRwLock::new(),
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: const-stable functions can only call other const-stable functions

error: could not compile `std` due to 3 previous errors

$ # same Cargo project as before
$ rustc -Vv | rg host
host: x86_64-unknown-linux-gnu

$ cargo build -Z build-std --target x86_64-unknown-linux-gnu && echo OK
   Compiling std
(..)
   Compiling proc_macro
(..)
OK

$ rustc -Vv
rustc 1.63.0-nightly (dc80ca78b 2022-06-21)
binary: rustc
commit-hash: dc80ca78b6ec2b6bba02560470347433bcd0bb3c
commit-date: 2022-06-21
host: x86_64-unknown-linux-gnu
release: 1.63.0-nightly
LLVM version: 14.0.5