Fix most ui tests on emscripten target

To fix the linker errors, we need to set the output extension to `.js` instead
of `.wasm`. Setting the output to a `.wasm` file puts Emscripten into standalone
mode which is effectively a distinct target. We need to set the runner to be
`node` as well.

This fixes most of the ui tests. I fixed a few more tests with simple problems:
- `intrinsics/intrinsic-alignment.rs` and `structs-enums/rec-align-u64.rs` --
Two `#[cfg]` macros match for Emscripten so we got a duplicate definition of
`mod m`.
- `issues/issue-12699.rs` -- Seems to hang so I disabled it
- `process/process-sigpipe.rs` -- Not expected to work on Emscripten so I
disabled it

src/bootstrap/src/core/config/config.rs

@@ -612,6 +612,9 @@ pub fn from_triple(triple: &str) -> Self {
         if triple.contains("-none") || triple.contains("nvptx") || triple.contains("switch") {
             target.no_std = true;
         }
+        if triple.contains("emscripten") {
+            target.runner = Some("node".into());
+        }
         target
     }
 }

src/tools/compiletest/src/runtest.rs

@@ -1643,7 +1643,9 @@ fn make_exe_name(&self) -> PathBuf {
         // double the length.
         let mut f = self.output_base_dir().join("a");
         // FIXME: This is using the host architecture exe suffix, not target!
-        if self.config.target.starts_with("wasm") {
+        if self.config.target.contains("emscripten") {
+            f = f.with_extra_extension("js");
+        } else if self.config.target.starts_with("wasm") {
             f = f.with_extra_extension("wasm");
         } else if self.config.target.contains("spirv") {
             f = f.with_extra_extension("spv");

src/tools/tidy/src/issues.txt

@@ -1520,7 +1520,6 @@ ui/issues/issue-12567.rs
 ui/issues/issue-12612.rs
 ui/issues/issue-12660.rs
 ui/issues/issue-12677.rs
-ui/issues/issue-12699.rs
 ui/issues/issue-12729.rs
 ui/issues/issue-12744.rs
 ui/issues/issue-12860.rs

src/tools/tidy/src/ui_tests.rs

@@ -17,7 +17,7 @@
 const ENTRY_LIMIT: u32 = 901;
 // FIXME: The following limits should be reduced eventually.
 
-const ISSUES_ENTRY_LIMIT: u32 = 1673;
+const ISSUES_ENTRY_LIMIT: u32 = 1672;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[
     "rs",     // test source files

tests/ui/intrinsics/intrinsic-alignment.rs

@@ -13,7 +13,6 @@ mod rusti {
 #[cfg(any(
     target_os = "android",
     target_os = "dragonfly",
-    target_os = "emscripten",
     target_os = "freebsd",
     target_os = "fuchsia",
     target_os = "hurd",

tests/ui/process/process-sigpipe.rs

@@ -15,11 +15,12 @@
 
 //@ ignore-vxworks no 'sh'
 //@ ignore-fuchsia no 'sh'
+//@ ignore-emscripten No threads
+//@ only-unix SIGPIPE is a unix feature
 
 use std::process;
 use std::thread;
 
-#[cfg(unix)]
 fn main() {
     // Just in case `yes` doesn't check for EPIPE...
     thread::spawn(|| {
@@ -34,8 +35,3 @@ fn main() {
     assert!(output.status.success());
     assert!(output.stderr.len() == 0);
 }
-
-#[cfg(not(unix))]
-fn main() {
-    // Not worried about signal masks on other platforms
-}

tests/ui/std/thread-sleep-ms.rs

@@ -1,6 +1,9 @@
 //@ run-pass
 //@ ignore-sgx not supported
+//@ ignore-emscripten
+// FIXME: test hangs on emscripten
 #![allow(deprecated)]
+#![allow(unused_imports)]
 
 use std::thread;
 

tests/ui/structs-enums/rec-align-u64.rs

@@ -33,7 +33,6 @@ struct Outer {
 #[cfg(any(
     target_os = "android",
     target_os = "dragonfly",
-    target_os = "emscripten",
     target_os = "freebsd",
     target_os = "fuchsia",
     target_os = "hurd",