Revert "Disable macOS on x86" (ocaml-flambda#3916)

Revert "Disable macOS on x86 (ocaml-flambda#3904)"

This reverts commit 0f1d0aa.

Author: spiessimon

configure.ac

@@ -1563,7 +1563,7 @@ AS_CASE([$host],
   [aarch64-*-darwin*],
     [has_native_backend=yes; arch=arm64; system=macosx],
   [x86_64-*-darwin*],
-    [AC_MSG_ERROR([macOS on x86 is no longer supported])],
+    [has_native_backend=yes; arch=amd64; system=macosx],
   [x86_64-*-mingw32*],
     [has_native_backend=yes; arch=amd64; system=mingw64],
   [aarch64-*-linux*],