Rollup merge of rust-lang#112753 - Zalathar:no-bless, r=clubby789

Don't try to auto-bless 32-bit `mir-opt` tests on ARM Mac hosts

rust-lang#112418 added special support for automatically blessing 32-bit output on 64-bit hosts, for the subset of `mir-opt` tests that are pointer-width-dependent.

This relies on the 64-bit host having some corresponding 32-bit target that can be built “easily”. For most 64-bit hosts this is fine, but ARM Macs don't have a corresponding 32-bit target. (There have never been 32-bit ARM Macs, and ARM Macs don't have the libraries needed for building `i686-apple-darwin`.)

There is an entry for `("i686-apple-darwin", "aarch64-apple-darwin")` in the list of corresponding 32-bit platforms, but this doesn't actually work on ARM Macs. Instead, the bootstrap invocation fails to build the necessary 32-bit target support, and nothing gets tested or blessed.

According to [this Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Can't.20bless.20any.20mir-opt.20tests.20on.20aarch64.20Mac/near/367109789), that mapping was only added because the author assumed it would work. But since it doesn't actually work on ARM Macs, the solution is to just remove that mapping.

With the mapping removed, ARM Macs still can't auto-bless 32-bit output (they will see a warning instead), but at least they can now bless the output of `mir-opt` tests that don't care about pointer width.

src/bootstrap/test.rs

@@ -44,7 +44,8 @@ const MIR_OPT_BLESS_TARGET_MAPPING: &[(&str, &str)] = &[
     ("i686-pc-windows-msvc", "x86_64-pc-windows-msvc"),
     ("i686-pc-windows-gnu", "x86_64-pc-windows-gnu"),
     ("i686-apple-darwin", "x86_64-apple-darwin"),
-    ("i686-apple-darwin", "aarch64-apple-darwin"),
+    // ARM Macs don't have a corresponding 32-bit target that they can (easily)
+    // build for, so there is no entry for "aarch64-apple-darwin" here.
 ];
 
 fn try_run(builder: &Builder<'_>, cmd: &mut Command) -> bool {