Add extraPlatforms for Rosetta 2 macOS

macOS systems with ARM64 can utilize a translation layer at
/Library/Apple/usr/libexec/oah to run x86_64 binaries. This change
makes Nix recognize that and it to "extra-platforms". Note that there
are two cases here since Nix could be built for either x86_64 or
aarch64. In either case, we can switch to the other architecture.
Unfortunately there is not a good way to prevent aarch64 binaries from
being run in x86_64 contexts or vice versa - programs can always
execute programs for the other architecture.

src/libstore/globals.cc

@@ -131,6 +131,28 @@ StringSet Settings::getDefaultSystemFeatures()
     return features;
 }
 
+StringSet Settings::getDefaultExtraPlatforms()
+{
+    if (std::string{SYSTEM} == "x86_64-linux" && !isWSL1())
+        return StringSet{"i686-linux"};
+#if __APPLE__
+    // Rosetta 2 emulation layer can run x86_64 binaries on aarch64
+    // machines. Note that we can’t force processes from executing
+    // x86_64 in aarch64 environments or vice versa since they can
+    // always exec with their own binary preferences.
+    else if (pathExists("/Library/Apple/usr/libexec/oah")) {
+        if (std::string{SYSTEM} == "x86_64-darwin")
+            return StringSet{"aarch64-darwin"};
+        else if (std::string{SYSTEM} == "aarch64-darwin")
+            return StringSet{"x86_64-darwin"};
+        else
+            return StringSet{};
+    }
+#endif
+    else
+        return StringSet{};
+}
+
 bool Settings::isExperimentalFeatureEnabled(const std::string & name)
 {
     auto & f = experimentalFeatures.get();

src/libstore/globals.hh

@@ -34,6 +34,8 @@ class Settings : public Config {
 
     StringSet getDefaultSystemFeatures();
 
+    StringSet getDefaultExtraPlatforms();
+
     bool isWSL1();
 
 public:
@@ -545,7 +547,7 @@ public:
 
     Setting<StringSet> extraPlatforms{
         this,
-        std::string{SYSTEM} == "x86_64-linux" && !isWSL1() ? StringSet{"i686-linux"} : StringSet{},
+        getDefaultExtraPlatforms(),
         "extra-platforms",
         R"(
           Platforms other than the native one which this machine is capable of