build: add explicit architecture check

When doing a unified cross-compile, the target will always exist, but
the architecture will not be the same.

Author: compnerd

CMakeLists.txt

@@ -953,7 +953,8 @@ if(SWIFT_BUILD_SYNTAXPARSERLIB OR SWIFT_BUILD_SOURCEKIT)
       set(SWIFT_LIBDISPATCH_CXX_COMPILER ${CMAKE_CXX_COMPILER})
     elseif(${CMAKE_SYSTEM_NAME} STREQUAL ${CMAKE_HOST_SYSTEM_NAME})
       if(CMAKE_SYSTEM_NAME STREQUAL Windows)
-        if(TARGET clang)
+        if(CMAKE_SYSTEM_PROCESSOR STREQUAL CMAKE_HOST_SYSTEM_PROCESSOR AND
+            TARGET clang)
           set(SWIFT_LIBDISPATCH_C_COMPILER
               $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
           set(SWIFT_LIBDISPATCH_CXX_COMPILER