Rollup merge of #145884 - clubby789:test-mcount, r=Mark-Simulacrum

Test `instrument-mcount` codegen

Closes #92109 by testing that a call to `mcount` is actually emitted

Author: Zalathar

tests/assembly-llvm/x86_64-mcount.rs

@@ -0,0 +1,24 @@
+//@ assembly-output: emit-asm
+//@ compile-flags: -Zinstrument-mcount=y -Cllvm-args=-x86-asm-syntax=intel
+
+//@ revisions: x86_64-linux
+//@[x86_64-linux] compile-flags: --target=x86_64-unknown-linux-gnu
+//@[x86_64-linux] needs-llvm-components: x86
+//@[x86_64-linux] only-x86_64-unknown-linux-gnu
+
+//@ revisions: x86_64-darwin
+//@[x86_64-darwin] compile-flags: --target=x86_64-apple-darwin
+//@[x86_64-darwin] needs-llvm-components: x86
+//@[x86_64-darwin] only-x86_64-apple-darwin
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: mcount_func:
+#[no_mangle]
+pub fn mcount_func() {
+    // CHECK: call mcount
+
+    std::hint::black_box(());
+
+    // CHECK: ret
+}