[gn] Use label_name rather than target_output_name for objects

Multiple targets in the same output directory can use the same
target_output_name. The typical example of that is having a shared
and a static library of the same, e.g. libc++.so and libc++.a.
When that's the case, the object files produced for each target
are going to conflict. Using the label_name avoids this conflict
since labels are guaranteed to be unique within a single BUILD.gn
file which corresponds to a single output directory.

Differential Revision: https://reviews.llvm.org/D60329

llvm-svn: 359494