Update CMakeLists.txt

Add a lowercase spelling for ARM64.  For some reason in CMake's infinite wisdom where it matches strings case insensitively, it performs a case sensitive match in this case.  :sigh:

Author: compnerd

CMakeLists.txt

@@ -605,7 +605,7 @@ if(SWIFT_HOST_VARIANT_ARCH)
 else()
   if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|AMD64")
     set(SWIFT_HOST_VARIANT_ARCH_default "x86_64")
-  elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64|ARM64")
+  elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64|ARM64|arm64")
     set(SWIFT_HOST_VARIANT_ARCH_default "aarch64")
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64")
     set(SWIFT_HOST_VARIANT_ARCH_default "powerpc64")