[MLton] MLton on arm64 (AArch64)

I found I needed the following patch to build it on arm64 Debian
unstable. Here's a log message, in case someone wants to commit it:

Add further support for arm64 (aarch64).

diff --git a/lib/stubs/mlton-stubs/mlton.sml b/lib/stubs/mlton-stubs/mlton.sml
index f494ff2..602806a 100644
--- a/lib/stubs/mlton-stubs/mlton.sml
+++ b/lib/stubs/mlton-stubs/mlton.sml
@@ -158,12 +158,13 @@ structure MLton: MLTON =
 
             structure Arch =
                struct
-                  datatype t = Alpha | AMD64 | ARM | HPPA | IA64 | m68k |
+                  datatype t = Alpha | AMD64 | ARM | ARM64 | HPPA | IA64 | m68k |
                                MIPS | PowerPC | PowerPC64 | S390 | Sparc | X86
 
                   val all = [(Alpha, "Alpha"),
                              (AMD64, "AMD64"),
                              (ARM, "ARM"),
+                             (ARM64, "ARM64"),
                              (HPPA, "HPPA"),
                              (IA64, "IA64"),
                              (m68k, "m68k"),
diff --git a/lib/stubs/mlton-stubs/platform.sig b/lib/stubs/mlton-stubs/platform.sig
index 507a8dd..f962656 100644
--- a/lib/stubs/mlton-stubs/platform.sig
+++ b/lib/stubs/mlton-stubs/platform.sig
@@ -9,7 +9,7 @@ signature MLTON_PLATFORM =
    sig
       structure Arch:
          sig
-            datatype t = Alpha | AMD64 | ARM | HPPA | IA64 | m68k |
+            datatype t = Alpha | AMD64 | ARM | ARM64 | HPPA | IA64 | m68k |
                          MIPS | PowerPC | PowerPC64 | S390 | Sparc | X86
 
             val fromString: string -> t option
diff --git a/mlton/main/main.fun b/mlton/main/main.fun
index 78dce8d..6b897ad 100644
--- a/mlton/main/main.fun
+++ b/mlton/main/main.fun
@@ -193,6 +193,7 @@ fun defaultAlignIs8 () =
          Alpha => true
        | AMD64 => true
        | ARM => true
+       | ARM64 => true
        | HPPA => true
        | IA64 => true
        | MIPS => true
diff --git a/runtime/platform/arm64.h b/runtime/platform/arm64.h
new file mode 100644
index 0000000..d056da7
--- /dev/null
+++ b/runtime/platform/arm64.h
@@ -0,0 +1 @@
+#define MLton_Platform_Arch_host "arm64"

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