return 0 to indicate failure to create the disassembler.  A library routine
should not assert and just let the caller handler the error.  For example
darwin's otool(1) will simply print an error if it ends up using a library
that is not configured for a target it wants:

% otool -tv ViewController.o
ViewController.o:
(__TEXT,__text) section
can't create arm llvm disassembler

This is much better than an abort which appears as a crash to the user or
even the assert when using a Debug+Asserts built library:

Assertion failed: (MAI && "Unable to create target asm info!"), function LLVMCreateDisasmCPU, file /Volumes/SandBox/llvm/lib/MC/MCDisassembler/Disassembler.cpp, line 47.

radr://12539918

llvm-svn: 176880