build: effectively revert 986beb7

This change broke the Windows builds for all the variants.  `-z defs` cannot be
used on all targets, particularly in environments where the stdlib is being
compiled for a foreign host.  This needs to be handled more carefully as I
mentioned on the PR itself.  In the mean time, revert it to get Windows building
again.

Author: compnerd

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -81,9 +81,7 @@ macro(swift_common_standalone_build_config_llvm product is_cross_compiling)
   # HACK: this ugly tweaking is to prevent the propagation of the flag from LLVM
   # into swift.  The use of this flag pollutes all targets, and we are not able
   # to remove it on a per-target basis which breaks cross-compilation.
-  if("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
-    string(REGEX REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
-  endif()
+  string(REGEX REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
 
   set(PACKAGE_VERSION "${LLVM_PACKAGE_VERSION}")
   string(REGEX REPLACE "([0-9]+)\\.[0-9]+(\\.[0-9]+)?" "\\1" PACKAGE_VERSION_MAJOR