Auto merge of #76378 - petrochenkov:lldtest, r=Mark-Simulacrum

rustbuild: Build tests with LLD if `use-lld = true` was passed

Addresses https://github.com/rust-lang/rust/pull/76127#discussion_r479932392.

Our test suite is generally ready to run with an explicitly specified linker (https://github.com/rust-lang/rust/pull/45191),
 so LLD specified with `use-lld = true` works as well.

Only 4 tests fail (on `x86_64-pc-windows-msvc`):
```
ui/panic-runtime/lto-unwind.rs
run-make-fulldeps/debug-assertions
run-make-fulldeps/foreign-exceptions
run-make-fulldeps/test-harness
```
All of them are legitimate issues with LLD (or at least with combination Rust+LLD) and manifest in segfaults on access to TLS (https://github.com/rust-lang/rust/pull/76127#issuecomment-683473325). UPD: These issues are caused by https://github.com/rust-lang/rust/issues/72145 and appear because I had `-Ctarget-cpu=native` set.

UPD: Further commits build tests with LLD for non-MSVC targets and propagate LLD to more places when `use-lld` is enabled.

src/bootstrap/lib.rs

@@ -844,7 +844,7 @@ impl Build {
     }
 
     /// Returns the path to the linker for the given target if it needs to be overridden.
-    fn linker(&self, target: TargetSelection, can_use_lld: bool) -> Option<&Path> {
+    fn linker(&self, target: TargetSelection) -> Option<&Path> {
         if let Some(linker) = self.config.target_config.get(&target).and_then(|c| c.linker.as_ref())
         {
             Some(linker)
@@ -857,18 +857,19 @@ impl Build {
             && !target.contains("msvc")
         {
             Some(self.cc(target))
-        } else if target.contains("msvc")
-            && can_use_lld
-            && self.config.use_lld
-            && self.build == target
-        {
-            // Currently we support using LLD directly via `rust.use_lld` option only with MSVC
+        } else if self.config.use_lld && !self.is_fuse_ld_lld(target) && self.build == target {
             Some(&self.initial_lld)
         } else {
             None
         }
     }
 
+    // LLD is used through `-fuse-ld=lld` rather than directly.
+    // Only MSVC targets use LLD directly at the moment.
+    fn is_fuse_ld_lld(&self, target: TargetSelection) -> bool {
+        self.config.use_lld && !target.contains("msvc")
+    }
+
     /// Returns if this target should statically link the C runtime, if specified
     fn crt_static(&self, target: TargetSelection) -> Option<bool> {
         if target.contains("pc-windows-msvc") {