[965] Replace various alignment code with a shared ALIGN() macro.

[965] Replace various alignment code with a shared ALIGN() macro.

In the process, fix some alignment issues:
- Scratch space allocation was aligned into units of 1KB, while the allocation
  wanted units of bytes, so we never allocated enough space for scratch.
- GRF register count was programmed as ALIGN(val - 1, 16) / 16 instead of
  ALIGN(val, 16) / 16 - 1, which overcounted for val != 16n+1.