Add absolute load address support for the DynamicLoader plugins

The POSIX linker generally reports the load bias for the loaded
libraries but in some case it is useful to handle a library based on
absolute load address. Example usecases:
* Windows linker uses absolute addresses
* Library list came from different source (e.g. /proc/<pid>/maps)

Differential revision: http://reviews.llvm.org/D12233

llvm-svn: 245834