Cleaned up the dynamic library open/getsymbol/close code to use abstracted

flags such that symbols can be searched for within a shared library if desired.
Platforms that support the RTLD_FIRST flag can still take advantage of their
quicker lookups, and other platforms can still get the same fucntionality
with a little extra work.

Also changed LLDB_CONFIG flags over to either being defined, or not being
defined to stay in line with current open source practices and to prepare for
using autoconf or cmake to configure LLDB builds.

llvm-svn: 125064