[mach-o]: Support deprecated -seg1addr alias for -image_base

Because NO LINKER MAY CHANGE. EVER. Even if it's a complete rewrite
from scratch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217572 91177308-0d34-0410-b5e6-96231b3b80d8