[Build] Add SWIFT_SYSTEM_NAME setting.

This lets us override the library subdir name when its derivation
isn't straightforwardly just the lower-case version of `CMAKE_SYSTEM_NAME`.

rdar://123380262

Author: al45tair

CMakeLists.txt

@@ -274,7 +274,15 @@ add_compile_definitions($<$<OR:$<COMPILE_LANGUAGE:C>,$<COMPILE_LANGUAGE:CXX>>:HA
 
 
 if(ENABLE_SWIFT)
-  set(INSTALL_TARGET_DIR "${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>" CACHE PATH "Path where the libraries will be installed")
+  if(NOT SWIFT_SYSTEM_NAME)
+    if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
+      set(SWIFT_SYSTEM_NAME macosx)
+    else()
+      set(SWIFT_SYSTEM_NAME "$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>")
+    endif()
+  endif()
+
+  set(INSTALL_TARGET_DIR "${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/${SWIFT_SYSTEM_NAME}" CACHE PATH "Path where the libraries will be installed")
   set(INSTALL_DISPATCH_HEADERS_DIR "${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/dispatch" CACHE PATH "Path where the headers will be installed for libdispatch")
   set(INSTALL_BLOCK_HEADERS_DIR "${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/Block" CACHE PATH "Path where the headers will be installed for the blocks runtime")
   set(INSTALL_OS_HEADERS_DIR "${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/os" CACHE PATH "Path where the os/ headers will be installed")