[libc++] Move the linker script generation step to CMake
Summary:
This allows the linker script generation to query CMake properties
(specifically the dependencies of libc++.so) instead of having to
carry these dependencies around manually in global variables. Notice
the removal of the LIBCXX_INTERFACE_LIBRARIES global variable.
Reviewers: phosek, EricWF
Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits
Tags: #libc
Differential Revision: https://reviews.llvm.org/D68343
llvm-svn: 374116