Make fake macOS host default to aarch64

Summary:
x86_64 macs are mostly a thing of the past now.

When `--fake-host=macos` is passed without `--fake-arch`, resolve the fake architecture to `aarch64`. Preserve every explicitly passed `--fake-arch` value, including `default`.

Reviewed By: 8Keep

Differential Revision: D116819642

fbshipit-source-id: 8f53e67c756719205b9065c0502766b99176efcb

Author: scottcao

app/buck2_client_ctx/src/common.rs

@@ -342,9 +342,10 @@ impl CommonBuildConfigurationOptions {
         }
     }
     pub fn host_arch_override(&self) -> HostArchOverride {
-        match &self.fake_arch {
-            Some(v) => *v,
-            None => HostArchOverride::Default,
+        match (&self.fake_arch, &self.fake_host) {
+            (Some(v), _) => *v,
+            (None, Some(HostPlatformOverride::MacOs)) => HostArchOverride::AArch64,
+            (None, _) => HostArchOverride::Default,
         }
     }
     pub fn host_xcode_version_override(&self) -> Option<String> {