[compiler-rt] [compiler-rt][ARM] Make ARMv4T assembly builtins interwork (PR #221725)

[Ties Stuij via llvm-commits]

stuij wrote:

@compnerd perhaps my explanation above muddled the waters. This patch fixes assembly builtins for both Arm and Thumb state.

Fixing this for Arm state is not an academic issue. Arm state v4t is a working variant and is built and tested with [arm-toolchain](https://github.com/arm/arm-toolchain). This is a pretty severe issue for an otherwise functioning backend variant. And we get the Thumb fix for free, so why not.

I myself maintain a Game Boy Advance toolchain on top of LLVM, and just for the GBA I know a number of people making Rust and Swift projects using LLVM besides C/C++ ones.

The reason I found out about this issue is because this caused a bug in my own code. I know of at least one other user of my toolchain who is suffering from this particular issue.

And I know of others that are still professionally maintaining LLVM based armv4 Linux variants that are used on routers and such.

https://github.com/llvm/llvm-project/pull/221725