Bug 1243918 - Don't clobber safeseh in msvcc.sh -clang-cl; r=glandium

Currently we remove the -SAFESEH that we added in bug 1242722 by
mistake which breaks x86 builds.

Upstream patch: https://github.com/atgreen/libffi/pull/225

js/src/ctypes/libffi/msvcc.sh

@@ -70,7 +70,6 @@ do
     ;;
     -clang-cl)
       cl="clang-cl"
-      safeseh=
       shift 1
     ;;
     -O0)