Allow later -z name=<int> args to override earlier args

Summary:
lld already gives later -z options precedence in getZFlag().

This matches the behavior of ld.bfd and ld.gold, where later options
override earlier ones. (I tested with -z max-page-size and -z stack-size.)

Reviewers: ruiu, espindola, grimar

Reviewed By: ruiu, grimar

Subscribers: grimar, emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D52526

llvm-svn: 343145