Rollup merge of #135001 - Kobzol:bootstrap-mcp-510, r=onur-ozkan

Allow using self-contained LLD in bootstrap

In #116278, I added a `"self-contained"` mode to the `rust.use-lld` bootstrap option, which was designed for using the built-in LLD for linking compiler artifacts. However, this was later reverted in #118810.

This PR brings the old logic back, which switches LLD in bootstrap from `-fuse-ld=lld` to [MCP510](rust-lang/compiler-team#510 way of passing linker flags to enable LLD (both external and self-contained). So this does two changes:
1) Goes from `-fuse-ld=lld` to MCP510
2) Actually makes it possible to use the self-contained LLD to compile compiler artifacts

Regarding the second commit: Since #86113, we have been passing `-fuse-ld=lld` as a target flag to all tests when `use-lld = true` is enabled. This kind of worked for all tests, since it was just a linker argument, which has bypassed any compiler checks, and probably resulted only in some warning if the given target linker didn't actually support LLD. However, after the first commit, some tests actually start failing with this approach:
```
error: linker flavor `gnu-lld-cc` is incompatible with the current target
   |
   = note: compatible flavors are: llbc, ptx
```
So the second commit removes the passing of LLD flags as target flags to tests. I don't think that it's a good idea to pass specific compiler flags to all tests unconditionally, tbh. The doctest command from #86113 doesn't go through compiletest anymore, and doctests should be quite a lot faster since #126245 in general.

CC `@the8472`

If someone has a beefy machine, it would be nice to test whether this doesn't regress test execution speed. How to do that:
1) Enable `rust.use-lld = true` and `rust.lld = true` in `config.toml`
2) Benchmark `./x test tests/ui --force-rerun` between `master` and this PR

Once this is tested in the wild, I would like to make the self-contained LLD the default in CI, hopefully to make CI builds faster.

r? `@onur-ozkan`

Author: matthiaskrgr

src/bootstrap/src/core/build_steps/test.rs

@@ -1893,7 +1893,6 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
 
         let mut targetflags = flags;
         targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
-        targetflags.extend(linker_flags(builder, compiler.host, LldThreads::No));
         for flag in targetflags {
             cmd.arg("--target-rustcflags").arg(flag);
         }

src/bootstrap/src/utils/helpers.rs

@@ -481,7 +481,20 @@ pub fn linker_flags(
 ) -> Vec<String> {
     let mut args = vec![];
     if !builder.is_lld_direct_linker(target) && builder.config.lld_mode.is_used() {
-        args.push(String::from("-Clink-arg=-fuse-ld=lld"));
+        match builder.config.lld_mode {
+            LldMode::External => {
+                args.push("-Clinker-flavor=gnu-lld-cc".to_string());
+                // FIXME(kobzol): remove this flag once MCP510 gets stabilized
+                args.push("-Zunstable-options".to_string());
+            }
+            LldMode::SelfContained => {
+                args.push("-Clinker-flavor=gnu-lld-cc".to_string());
+                args.push("-Clink-self-contained=+linker".to_string());
+                // FIXME(kobzol): remove this flag once MCP510 gets stabilized
+                args.push("-Zunstable-options".to_string());
+            }
+            LldMode::Unused => unreachable!(),
+        };
 
         if matches!(lld_threads, LldThreads::No) {
             args.push(format!(