Update TestLoadUnload to use base Makefile.

Summary:
The current Makefile scheme only allows one dylib to be specified in each make
invocation, so TestLoadUnload had a custom Makefile that's unrelated to the
base Makefile.rules. This change uses recursive make invocations to bypass the
single dylib restriction. See D11202 for more context.

Reviewers: clayborg

Subscribers: chaoren, lldb-commits

Differential Revision: http://reviews.llvm.org/D11367

llvm-svn: 242813