reduce threads spawned by ui-tests

the test harness already spawns enough tests for all cores, individual
tests should keep their own threading to a minimum to avoid context switch
overhead

some tests fail with 1 CGU, so explicit compile flags have been added
to keep their old behavior

# Conflicts:
#	src/test/ui/asm/sym.rs

Author: the8472

src/bootstrap/test.rs

@@ -1156,13 +1156,15 @@ note: if you're sure you want to do this, please open an issue as to why. In the
         hostflags.push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
         if builder.is_fuse_ld_lld(compiler.host) {
             hostflags.push("-Clink-args=-fuse-ld=lld".to_string());
+            hostflags.push("-Clink-arg=-Wl,--threads=1".to_string());
         }
         cmd.arg("--host-rustcflags").arg(hostflags.join(" "));
 
         let mut targetflags = flags;
         targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
         if builder.is_fuse_ld_lld(target) {
             targetflags.push("-Clink-args=-fuse-ld=lld".to_string());
+            targetflags.push("-Clink-arg=-Wl,--threads=1".to_string());
         }
         cmd.arg("--target-rustcflags").arg(targetflags.join(" "));
 

src/test/ui/asm/sym.rs

@@ -1,4 +1,5 @@
 // min-llvm-version: 10.0.1
+// compile-flags: -C codegen-units=2
 // only-x86_64
 // only-linux
 // run-pass

src/test/ui/issues/issue-69225-SCEVAddExpr-wrap-flag.rs

@@ -1,5 +1,6 @@
 // run-fail
 // compile-flags: -C opt-level=3
+// min-llvm-version: 11.0
 // error-pattern: index out of bounds: the len is 0 but the index is 16777216
 // ignore-wasm no panic or subprocess support
 // ignore-emscripten no panic or subprocess support

src/test/ui/linkage-attr/linkage-detect-extern-generated-name-collision.rs

@@ -4,6 +4,7 @@
 
 // build-fail
 // aux-build:def_colliding_external.rs
+// compile-flags: -Ccodegen-units=16
 
 extern crate def_colliding_external as dep1;
 

src/test/ui/linkage-attr/linkage-detect-local-generated-name-collision.rs

@@ -1,5 +1,5 @@
 // build-fail
-
+// compile-flags: -Ccodegen-units=16
 #![feature(linkage)]
 
 mod dep1 {

src/tools/compiletest/src/runtest.rs

@@ -1952,6 +1952,7 @@ impl<'test> TestCx<'test> {
                 if !self.props.compile_flags.iter().any(|s| s.starts_with("--error-format")) {
                     rustc.args(&["--error-format", "json"]);
                 }
+                rustc.arg("-Ccodegen-units=1");
                 rustc.arg("-Zui-testing");
                 rustc.arg("-Zdeduplicate-diagnostics=no");
                 rustc.arg("-Zemit-future-incompat-report");