[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

llvm-svn: 358915
GitOrigin-RevId: e9a2d1c3921a19d4082eb132e055dc40c563e071
2 files changed