Add a test that #[used] makes it through to the linker on macos

Author: thomcc

src/test/run-make-fulldeps/used-cdylib-macos/Makefile

@@ -0,0 +1,11 @@
+-include ../tools.mk
+
+# only-macos
+#
+# This checks that `#[used]` passes through to the linker on
+# darwin. This is subject to change in the future, see
+# https://github.com/rust-lang/rust/pull/93718 for discussion
+
+all:
+	$(RUSTC) -Copt-level=3 dylib_used.rs
+	nm $(TMPDIR)/libdylib_used.dylib | $(CGREP) VERY_IMPORTANT_SYMBOL

src/test/run-make-fulldeps/used-cdylib-macos/dylib_used.rs

@@ -0,0 +1,4 @@
+#![crate_type = "cdylib"]
+
+#[used]
+static VERY_IMPORTANT_SYMBOL: u32 = 12345;