emscripten: Lower max_align_t from 16 to 8 bytes

See: llvm/llvm-project@d1a96e9.

(backport <rust-lang#3962>)
(cherry picked from commit 99035d7)

Author: kleisauke

libc-test/build.rs

@@ -2945,10 +2945,6 @@ fn test_emscripten(target: &str) {
             ty if ty.starts_with("epoll") => true,
             ty if ty.starts_with("signalfd") => true,
 
-            // FIXME: Lowered from 16 to 8 bytes in
-            // llvm/llvm-project@d1a96e9
-            "max_align_t" => true,
-
             // FIXME: The size has been changed due to time64
             "utimbuf" | "timeval" | "timespec" | "rusage" | "itimerval" | "sched_param"
             | "stat" | "stat64" | "shmid_ds" | "msqid_ds" => true,

src/unix/linux_like/emscripten/align.rs

@@ -38,9 +38,9 @@ macro_rules! expand_align {
             }
 
             #[allow(missing_debug_implementations)]
-            #[repr(align(16))]
+            #[repr(align(8))]
             pub struct max_align_t {
-                priv_: [f64; 4]
+                priv_: [f64; 3]
             }
 
         }