Fix flags when using clang as linker for Fuchsia

Don't add C runtime or set dynamic linker when linking with clang for
Fuchsia. Clang already does this for us.

Author: tmandry

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1551,12 +1551,21 @@ fn crt_objects_fallback(sess: &Session, crate_type: CrateType) -> bool {
 fn add_pre_link_objects(
     cmd: &mut dyn Linker,
     sess: &Session,
+    flavor: LinkerFlavor,
     link_output_kind: LinkOutputKind,
     self_contained: bool,
 ) {
+    // FIXME: we are currently missing some infra here (per-linker-flavor CRT objects),
+    // so Fuchsia has to be special-cased.
     let opts = &sess.target;
-    let objects =
-        if self_contained { &opts.pre_link_objects_fallback } else { &opts.pre_link_objects };
+    let empty = Default::default();
+    let objects = if self_contained {
+        &opts.pre_link_objects_fallback
+    } else if !(sess.target.os == "fuchsia" && flavor == LinkerFlavor::Gcc) {
+        &opts.pre_link_objects
+    } else {
+        &empty
+    };
     for obj in objects.get(&link_output_kind).iter().copied().flatten() {
         cmd.add_object(&get_object_file_path(sess, obj, self_contained));
     }
@@ -1881,7 +1890,7 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
     // ------------ Object code and libraries, order-dependent ------------
 
     // Pre-link CRT objects.
-    add_pre_link_objects(cmd, sess, link_output_kind, crt_objects_fallback);
+    add_pre_link_objects(cmd, sess, flavor, link_output_kind, crt_objects_fallback);
 
     add_linked_symbol_object(
         cmd,
@@ -2018,7 +2027,10 @@ fn add_order_independent_options(
 
     add_link_script(cmd, sess, tmpdir, crate_type);
 
-    if sess.target.os == "fuchsia" && crate_type == CrateType::Executable {
+    if sess.target.os == "fuchsia"
+        && crate_type == CrateType::Executable
+        && flavor != LinkerFlavor::Gcc
+    {
         let prefix = if sess.opts.unstable_opts.sanitizer.contains(SanitizerSet::ADDRESS) {
             "asan/"
         } else {

compiler/rustc_target/src/spec/fuchsia_base.rs

@@ -1,6 +1,11 @@
 use crate::spec::{crt_objects, cvs, LinkOutputKind, LinkerFlavor, LldFlavor, TargetOptions};
 
 pub fn opts() -> TargetOptions {
+    // This mirrors the linker options provided by clang. We presume lld for
+    // now. When using clang as the linker it will supply these options for us,
+    // so we only list them for ld/lld.
+    //
+    // https://github.com/llvm/llvm-project/blob/db9322b2066c55254e7691efeab863f43bfcc084/clang/lib/Driver/ToolChains/Fuchsia.cpp#L31
     let pre_link_args = TargetOptions::link_args(
         LinkerFlavor::Ld,
         &[