[Support][Endian] Overload += and -=

This patch is to overload operator+= and operator-= for
{u}{little}{big}{16,32,64}_t.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217637 91177308-0d34-0410-b5e6-96231b3b80d8