Add --print host-triple

People often parse `-vV` output to get to the host triple, which is
annoying to do. It's easier to just get it directly.

I called it "host-triple" instead of "host" because it's clearer that
it's just the target name.

Author: Noratrieb

Diff for: compiler/rustc_driver_impl/src/lib.rs

@@ -734,6 +734,7 @@ fn print_crate_info(
                 targets.sort_unstable();
                 println_info!("{}", targets.join("\n"));
             }
+            HostTriple => println_info!("{}", rustc_session::config::host_triple()),
             Sysroot => println_info!("{}", sess.sysroot.display()),
             TargetLibdir => println_info!("{}", sess.target_tlib_path.dir.display()),
             TargetSpec => {

Diff for: compiler/rustc_session/src/config.rs

@@ -769,6 +769,7 @@ pub struct PrintRequest {
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum PrintKind {
     FileNames,
+    HostTriple,
     Sysroot,
     TargetLibdir,
     CrateName,
@@ -1863,6 +1864,7 @@ fn collect_print_requests(
         ("crate-name", PrintKind::CrateName),
         ("deployment-target", PrintKind::DeploymentTarget),
         ("file-names", PrintKind::FileNames),
+        ("host-triple", PrintKind::HostTriple),
         ("link-args", PrintKind::LinkArgs),
         ("native-static-libs", PrintKind::NativeStaticLibs),
         ("relocation-models", PrintKind::RelocationModels),

Diff for: tests/ui/invalid-compile-flags/print.stderr

@@ -1,4 +1,4 @@
 error: unknown print request: `yyyy`
   |
-  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
+  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `host-triple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
 

Diff for: tests/ui/print-request/host.rs

@@ -0,0 +1,5 @@
+//@ only-x86_64-unknown-linux-gnu
+//@ compile-flags: --print host-triple
+//@ check-pass
+
+fn main() {}

Diff for: tests/ui/print-request/host.stdout

@@ -0,0 +1 @@
+x86_64-unknown-linux-gnu