backport of https://github.com/apple/swift-corelibs-libdispatch/pull/559

Index: cmake/modules/SwiftSupport.cmake
--- cmake/modules/SwiftSupport.cmake.orig
+++ cmake/modules/SwiftSupport.cmake
@@ -27,6 +27,8 @@ function(get_swift_host_arch result_var_name)
     set("${result_var_name}" "armv7" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "armv7l")
     set("${result_var_name}" "armv7" PARENT_SCOPE)
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "amd64")
+    set("${result_var_name}" "amd64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "AMD64")
     set("${result_var_name}" "x86_64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "IA64")
