cortex-m-rt: Remove LR push, to ensure the stack is 8-byte aligned.

[Dirbaio]

This was causing incorrect execution of code optimized with the assumption the stack is 8-byte aligned.

Alternate version of #463

• Remove instead of fix the sentinel/fake frame.
• Remove code initializing LR, since it's now clobbered by the bl main anyway.
• Remove the .cfi directives, since Reset now has no correct CFI info. I think this is the "correct" thing to do here.
• Initialize the frame pointer in R7 (suggestion from @jamesmunns)

[jamesmunns]

Love it,

[adamgreig]

On balance I want to go this route, instead of #463. Thanks for preparing all the PRs @Dirbaio, and thanks to everyone else who helped investigate, especially @Dirbaio and @jamesmunns, and @peter9477 for spotting the issue in the first place.

There's a lot of useful context, explanation, and research in the #463 thread, though, so future readers should check it out.

Apologies in advance to the probe-run team; I think in the end it's best to go directly to the solution we want to stick with, and hopefully it's not too bad to patch up probe-run to eliminate the warning it will start emitting.

bors r+

[bors]

Build succeeded:

• ci-linux (1.59.0)
• ci-linux (stable)
• clippy
• hil-compile-rtt
• hil-qemu
• rt-ci-linux (1.59.0)
• rt-ci-linux (stable)
• rt-ci-other-os (macOS-latest)
• rt-ci-other-os (windows-latest)
• rustfmt