`thumbv6m-none-eabi` inline asm `clobber_abi("C")` fails with `register 'LR' allocated for constraint '{r14}' does not match required type`

The following minimized example fails to compile on the latest nightly, but works on stable and the previous nightly:

#![no_std]

#[no_mangle]
pub fn test() {
    unsafe { core::arch::asm!("nop", clobber_abi("C")) }
}

#[panic_handler]
fn panic(_: &core::panic::PanicInfo) -> ! {
    loop {}
}

Current nightly:

❯ rustc +nightly-2026-09-22 --version
rustc 1.100.0-nightly (1303417c4 2026-09-21)
                                                                                                                                                                                      
❯ rustc +nightly-2026-09-22 --target thumbv6m-none-eabi --crate-type staticlib -o repro.a repro.rs
error: register 'LR' allocated for constraint '{r14}' does not match required type
 --> repro.rs:6:32
  |
6 |     unsafe { core::arch::asm!("nop", clobber_abi("C")) }
  |                                ^^^

error: aborting due to 1 previous error

Prior nightly:

❯ rustc +nightly-2026-09-21 --version                                                             
rustc 1.100.0-nightly (bba531001 2026-09-20)

❯ rustc +nightly-2026-09-21 --target thumbv6m-none-eabi --crate-type staticlib -o repro.a repro.rs

Stable:

❯ rustc +stable --version                                                             
rustc 1.98.1 (48a229cea 2026-09-01)
                                                                                                                                                                                          
❯ rustc +stable --target thumbv6m-none-eabi --crate-type staticlib -o repro.a repro.rs

This can also be reproduced with the examples of embassy-rp and zeptos, and likely anything that calls into the RP2040 boot ROM for flash operations.