Support thumbv4t-none-eabi and thumbv5te-none-eabi

[thejpster]

There are targets for ARMv4T and ARMv5TE that default to Thumb mode and not Arm mode.

Support is relatively easy - all the assembly routines should use a .arm directive to indicate that they should be assembled to A32 machine code rather than T32 machine code. Some functions contain inline assembly that isn't possible in older Thumb variants, and those are expressly marked as being in Arm mode.

I also took the chance to clean up the interrupt disable/enable routines, splitting the atomic fences from the actual inline assembly.

[robamu]

Oh, these targets are not documented in the rustc docs? Might be a good idea to update https://doc.rust-lang.org/rustc/platform-support/arm-none-eabi.html . Should i look into that? We could update the maintainer list for https://doc.rust-lang.org/rustc/platform-support/armv5te-none-eabi.html and/or https://doc.rust-lang.org/rustc/platform-support/armv4t-none-eabi.html while we're at it.

[thejpster]

They are on https://doc.rust-lang.org/rustc/platform-support/arm-none-eabi.html as Tier 3 targets.

I don’t think I’m ready to volunteer to maintain the targets, but I will ping the maintainers on Zulip to all about atomics.

[thejpster]

@QuinnPainter, I see you’re the maintainer of ARMv5TE bare metal support. We’ve added the target to our little aarch32 library here, which might be of interest. I also wanted to ask about atomics on this platform. I have a Zulip thread at https://rust-lang.zulipchat.com/#narrow/channel/242906-t-compiler.2Farm/topic/__sync_lock_test_and_set_4.20on.20Armv5TE if you have thoughts on that.