[libcxx] Update gen_link_script.py to support different input and output

This enables the use of this script from other build systems like
GN which don't support post-build actions as well as for static
archives.

Differential Revision: https://reviews.llvm.org/D60309

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358915 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed