gpu-compute: mark functions with override if replacing virtual

The clang compiler is more stringent than the recent versions of
GCC when dealing with overrides. This changeset adds the specifier
to the methods which need it to silence the compiler.

gpu-compute, hsail: make the PC a byte address, not an instruction index

currently the PC is incremented on an instruction granularity, and not as an
instruction's byte address. machine ISA instructions assume the PC is a byte
address, and is incremented accordingly. here we make the GPU model, and the
HSAIL instructions treat the PC as a byte address as well.

gpu-compute: move disassemle() implementation to GPUStaticInst

gpu-compute, arch: add some methods to the base inst classes for ISA support

gpu-compute: AMD's baseline GPU model