[libcxx] Use relative path for libc++ library when generating script

This addresses the issue introduced in D60309 which leads to linker
scripts being generated with absolute paths.

Differential Revision: https://reviews.llvm.org/D61039

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359045 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed