Auto merge of #142443 - matthiaskrgr:rollup-l1l6d0v, r=matthiaskrgr

Rollup of 9 pull requests

Successful merges:

 - rust-lang/rust#128425 (Make `missing_fragment_specifier` an unconditional error)
 - rust-lang/rust#135927 (retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features)
 - rust-lang/rust#140770 (add `extern "custom"` functions)
 - rust-lang/rust#142176 (tests: Split dont-shuffle-bswaps along opt-levels and arches)
 - rust-lang/rust#142248 (Add supported asm types for LoongArch32)
 - rust-lang/rust#142267 (assert more in release in `rustc_ast_lowering`)
 - rust-lang/rust#142274 (Update the stdarch submodule)
 - rust-lang/rust#142276 (Update dependencies in `library/Cargo.lock`)
 - rust-lang/rust#142308 (Upgrade `object`, `addr2line`, and `unwinding` in the standard library)

Failed merges:

 - rust-lang/rust#140920 (Extract some shared code from codegen backend target feature handling)

r? `@ghost`
`@rustbot` modify labels: rollup

try-job: aarch64-apple
try-job: x86_64-msvc-1
try-job: x86_64-gnu
try-job: dist-i586-gnu-i586-i686-musl
try-job: test-various

src/abi/mod.rs

@@ -51,6 +51,11 @@ pub(crate) fn conv_to_call_conv(
         CanonAbi::Rust | CanonAbi::C => default_call_conv,
         CanonAbi::RustCold => CallConv::Cold,
 
+        // Functions with this calling convention can only be called from assembly, but it is
+        // possible to declare an `extern "custom"` block, so the backend still needs a calling
+        // convention for declaring foreign functions.
+        CanonAbi::Custom => default_call_conv,
+
         CanonAbi::X86(x86_call) => match x86_call {
             X86Call::SysV64 => CallConv::SystemV,
             X86Call::Win64 => CallConv::WindowsFastcall,