Make build-config magic use memfd by default.

crates/runtime/src/traphandlers/unix.rs

@@ -54,14 +54,7 @@ pub unsafe fn platform_init() {
     // Sometimes we need to handle SIGBUS too:
     // - On ARM, handle Unaligned Accesses.
     // - On Darwin, guard page accesses are raised as SIGBUS.
-    // - With the MemFD allocator, heap growth is controlled by
-    //   ftruncate'ing an mmap'd file, and so out-of-bounds accesses
-    //   are raised as SIGBUS.
-    if cfg!(target_arch = "arm")
-        || cfg!(target_os = "macos")
-        || cfg!(target_os = "freebsd")
-        || cfg!(feature = "memfd-allocator")
-    {
+    if cfg!(target_arch = "arm") || cfg!(target_os = "macos") || cfg!(target_os = "freebsd") {
         register(&mut PREV_SIGBUS, libc::SIGBUS);
     }
 }