i965: Add annotation data structure and support code.

i965: Add annotation data structure and support code.

Will be used to print disassembly after jump targets are set and
instructions are compacted, while still retaining higher-level IR
annotations and basic block information.

An array of 'struct annotation' will live along side the generated
assembly. The generators will populate the array with their IR
annotations, and basic block pointers if the instructions began or ended
a basic block pointer.

We'll then update the instruction offset when we compact instructions
and then using the annotations print the disassembly.

Reviewed-by: Eric Anholt <eric@anholt.net>