python: Properly initialize before trying to create disasm

As the "LLVMInitializeAll*" functions are not available as symbols in
the shared library they can't be used, and as a workaround a list of
the targets is kept and the individual symbols tried. As soon as the
"All"-functions are changed to proper symbols (as opposed to static
inlines in the headers) this hack will be replace with simple calls
to the corresponding "LLVMInitializeAll*" functions.

Reviewed By: indygreg

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1879