[cmake] Support OpenBSD amd64 architecture name.

x86_64 is spelled "amd64" on this platform. Return "amd64", for
consistency with Swift and other projects. For Windows, return
"x86_64".

Author: 3405691582

cmake/modules/SwiftSupport.cmake

@@ -27,8 +27,12 @@ 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("${result_var_name}" "x86_64" PARENT_SCOPE)
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "amd64|AMD64")
+    if(CMAKE_SYSTEM_NAME MATCHES "Windows")
+      set("${result_var_name}" "x86_64" PARENT_SCOPE)
+    else()
+      set("${result_var_name}" "amd64" PARENT_SCOPE)
+    endif()
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "IA64")
     set("${result_var_name}" "itanium" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86")