Unaligned memory access instructions are generated on thumbv7em-none-eabi target

In some cases, unaligned memory access instructions are generated for the thumbv7em-none-eabi
embedded target. This crashes the program with a hard-fault error interrupt on thumbv7em MCU
variants like the XMC4800 that do not support unaligned memory access. Whether an thumbv7em MCU
supports unaligned memory access is indicated by by the UNALIGN_TRP bit in the CCR register,
see ARMv7 ARM.

This issue can be worked around by setting the rustflag target-feature=+strict-align though .cargo/config.tomlbut at the same time, this causes the following warning:

warning: unknown and unstable feature specified for `-Ctarget-feature`: `strict-align`
  |
  = note: it is still passed through to the codegen backend, but use of this feature might be
    unsound and the behavior of this feature can change in the future
  = help: consider filing a feature request

The following code snippet can trigger this issue in the derived Default implementation:

#[derive(Default)]
pub struct Config {
    pub sys_divider: u8,
    pub cpu_divider: bool,
    pub ccu_divider: bool,
    pub pb_divider: bool,
    pub ndiv: u8,
    pub pdiv: u8,
    pub kdiv: u8,
}

fn init_clock() {
    // Calling default on Config causes a hard-fault.
    let config = Config::default();
    core::hint::black_box(&config);
}

From this code, the following invalid unaligned store-word instruction is generated:

str.w   r0, [sp, #3] // The stack pointer `sp` is aligned correctly

The full minimal example together with the generated assembly code can be found at:
https://github.com/maximilian-maisel-bl/rust-thumbv7em-align-bug

A possible fix could be setting the strict align flag in the thumbv7em-none-eabi target spec file
similar to the changes made by #58060 for aarch64-unknown-none.

This bug might be related to #82945

Used rust version: rustc 1.87.0-nightly (617aad8c2 2025-02-24)

Start with the thumbv7em-none-eabi target spec and compare the target-feature changes made for aarch64-unknown-none in #58060. Use the linked minimal example and generated assembly to verify that the derived Default case no longer emits an unaligned store, and check whether the strict-align warning is resolved.