Skip PLT for built-in functions

Implementation inspired by:
https://reviews.llvm.org/D42217
https://reviews.llvm.org/D42216

Author: GabrielMajeri

src/librustc_codegen_llvm/context.rs

@@ -208,6 +208,13 @@ pub unsafe fn create_module(
         llvm::LLVMRustSetModulePIELevel(llmod);
     }
 
+    // If skipping the PLT is enabled, we need to add some module metadata
+    // to ensure intrinsic calls don't use it.
+    if sess.opts.debugging_opts.no_plt.unwrap_or(true) {
+        let avoid_plt = "RtLibUseGOT\0".as_ptr() as *const _;
+        llvm::LLVMRustAddModuleFlag(llmod, avoid_plt, 1);
+    }
+
     llmod
 }
 

src/test/codegen/no-plt.rs

@@ -22,3 +22,6 @@ extern "C" {
 pub unsafe fn call_through_plt() -> *mut u8 {
     getenv(b"\0".as_ptr())
 }
+
+// Ensure intrinsics also skip the PLT
+// CHECK: !"RtLibUseGOT"