[libc++abi] Do not share an object library to create the static/shared libraries

This change is similar to r356150, with the same motivation. The
only difference is that the method used to merge libunwind.a and
libc++abi.a had to be changed to use the same approach as libc++
since we no longer produce object libraries that could be linked
together as we did before. We reuse the libc++ script for merging
archives to avoid duplication between the two projects.

Differential Revision: https://reviews.llvm.org/D60173

llvm-svn: 357635