Create _lldb python symlink correctly when LLVM_LIBDIR_SUFFIX is used

Summary:
Do not assume that liblldb.so is located in $(lldb -P)/../../../lib
when creating the _lldb python symlink. Instead, use the path passed
to LLVM_LIBDIR_SUFFIX, defaulting to $(lldb -P)/../../../lib when this
variable is not set.

Reviewers: vharron, emaste, zturner

Subscribers: zturner, labath, lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D19067

llvm-svn: 267462