build: ensure that Windows is built without PIC

PIC on windows does not make sense.  All code is position independent.
Currently clang and LLVM do the wrong thing and generate ELF style PIC
code on Windows when `-fPIC` is used.  Add `-fno-pic` to disable that
when cross-compiling to Windows.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -230,6 +230,7 @@ function(_add_variant_c_compile_flags)
       list(APPEND result "-D_MDd")
       list(APPEND result -Xclang;--dependent-lib=msvcrtd)
     endif()
+    list(APPEND result -fno-pic)
   endif()
 
   if(CFLAGS_ENABLE_ASSERTIONS)