test suite: add ARM case to ABI-required target feature check for -Ctarget-cpu

tests/ui/target-feature/abi-required-target-feature-missing-in-target-cpu.arm.stderr

@@ -0,0 +1,7 @@
+warning: target feature `fpregs` must be enabled to ensure that the ABI of the current target can be implemented correctly
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
+warning: 1 warning emitted
+

tests/ui/target-feature/abi-required-target-feature-missing-in-target-cpu.rs

@@ -0,0 +1,23 @@
+//! Sometimes `-Ctarget-cpu` can *disable* target features that would by default be enabled on the
+//! current target. Ensure that we catch the case where those target features are important for the
+//! ABI.
+
+//@ compile-flags: --crate-type=lib
+//@ revisions: x86 arm
+//@[x86] compile-flags: --target=i686-unknown-linux-gnu -Ctarget-cpu=pentium
+//@[x86] needs-llvm-components: x86
+//@[arm] compile-flags: --target=armv8r-none-eabihf -Ctarget-cpu=cortex-r4
+//@[arm] needs-llvm-components: arm
+
+// For now this is just a warning.
+//@ build-pass
+//@ ignore-backends: gcc
+//@ add-minicore
+
+#![feature(no_core)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+//~? WARN must be enabled to ensure that the ABI of the current target can be implemented correctly