[LLDB] Remove standalone build dep on llvm-strip

When building standalone, since llvm-strip is a symlink, it is created
using add_custom_command/add_custom_target which cannot be exported, and
thus cannot be depended on by lldb.

Patch by: Gwen Mittertreiner

Differential revision: https://reviews.llvm.org/D68614

llvm-svn: 374229