[ELF] Fix driver bug.

GNU LD-comptaible driver wrongly requires a space after '=' for a few
options such as "-init=<symbol>" or "-entry=<symbol>". This patch is
to fix that bug and add a few tests for it.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205693 91177308-0d34-0410-b5e6-96231b3b80d8