add end-to-end test of custom target using rust-lld

starting from the x86_64-unknown-linux-gnu specs, we add the lld linker
flavor and self-contained linker component

Author: lqd

tests/run-make/rust-lld-custom-target/Makefile

@@ -0,0 +1,7 @@
+include ../tools.mk
+
+# needs-rust-lld
+# only-x86_64-unknown-linux-gnu
+all:
+	RUSTC_LOG=rustc_codegen_ssa::back::link=info $(RUSTC) --crate-type cdylib --target custom-target.json -Clink-args=-Wl,-v lib.rs 2> $(TMPDIR)/output.txt
+	$(CGREP) -e "^LLD [0-9]+\.[0-9]+\.[0-9]+" < $(TMPDIR)/output.txt

tests/run-make/rust-lld-custom-target/custom-target.json

@@ -0,0 +1,57 @@
+{
+  "arch": "x86_64",
+  "cpu": "x86-64",
+  "crt-static-respected": true,
+  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
+  "dynamic-linking": true,
+  "env": "gnu",
+  "has-rpath": true,
+  "has-thread-local": true,
+  "link-self-contained": {
+    "components": [
+      "linker"
+    ]
+  },
+  "linker-flavor": "gnu-lld-cc",
+  "llvm-target": "x86_64-unknown-linux-gnu",
+  "max-atomic-width": 64,
+  "os": "linux",
+  "plt-by-default": false,
+  "position-independent-executables": true,
+  "pre-link-args": {
+    "gnu-cc": [
+      "-m64"
+    ],
+    "gnu-lld-cc": [
+      "-m64"
+    ]
+  },
+  "relro-level": "full",
+  "stack-probes": {
+    "kind": "inline-or-call",
+    "min-llvm-version-for-inline": [
+      16,
+      0,
+      0
+    ]
+  },
+  "static-position-independent-executables": true,
+  "supported-sanitizers": [
+    "address",
+    "cfi",
+    "leak",
+    "memory",
+    "thread",
+    "safestack"
+  ],
+  "supported-split-debuginfo": [
+    "packed",
+    "unpacked",
+    "off"
+  ],
+  "supports-xray": true,
+  "target-family": [
+    "unix"
+  ],
+  "target-pointer-width": "64"
+}

tests/run-make/rust-lld-custom-target/lib.rs

@@ -0,0 +1,9 @@
+// Test linking using `cc` with `rust-lld`, using a custom target with features described in MCP 510
+// see https://github.com/rust-lang/compiler-team/issues/510 for more info:
+//
+// Starting from the `x86_64-unknown-linux-gnu` target spec, we add the following options:
+// - a linker-flavor using lld via a C compiler
+// - the self-contained linker component is enabled
+
+#![feature(no_core)]
+#![no_core]