Update CMakeLists.txt

Also correct the spelling for Darwin which uses the arm64 spelling instead of aarch64.

Author: compnerd

CMakeLists.txt

@@ -606,7 +606,11 @@ else()
   if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|AMD64")
     set(SWIFT_HOST_VARIANT_ARCH_default "x86_64")
   elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64|ARM64|arm64")
-    set(SWIFT_HOST_VARIANT_ARCH_default "aarch64")
+    if(SWIFT_HOST_VARIANT_SDK_default STREQUAL OSX)
+      set(SWIFT_HOST_VARIANT_ARCH_default "arm64")
+    else()
+      set(SWIFT_HOST_VARIANT_ARCH_default "aarch64")
+    endif()
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64")
     set(SWIFT_HOST_VARIANT_ARCH_default "powerpc64")
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64le")