Add to the disassembler C API an option to print the disassembled

Add to the disassembler C API an option to print the disassembled
instructions in the assembly code variant if one exists.

The intended use for this is so tools like lldb and darwin's otool(1)
can be switched to print Intel-flavored disassembly.

I discussed extensively this API with Jim Grosbach and we feel
while it may not be fully general, in reality there is only one syntax
for each assembly with the exception of X86 which has exactly
two for historical reasons.

rdar://10989182

llvm-svn: 170477