Rollup merge of #103989 - arlosi:arm32-panic, r=Amanieu

Fix build of std for thumbv7a-pc-windows-msvc

Attempting to build std for the tier-3 target `thumbv7a-pc-windows-msvc` fails with the following error:
```
Building stage1 std artifacts (x86_64-pc-windows-msvc -> thumbv7a-pc-windows-msvc)
..
LLVM ERROR: WinEH not implemented for this target
error: could not compile `panic_unwind`
```

EH (unwinding) is not supported by LLVM for 32 bit arm msvc targets. This changes panic unwind to use the dummy implementation for `thumbv7a-pc-windows-msvc`.