swr/rast: Cleanup and generalize gen_archrast

swr/rast: Cleanup and generalize gen_archrast

- Update meson.build
  - Includes current_build_dir() fix
    meson/swr: replace hard-coded path with current_build_dir()

    Fixes: 93cd9905c8fbb98985ae "swr/rast: Cleanup and generalize gen_archrast"
    Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
    Reviewed-by: Alok Hota <alok.hota@intel.com>
    Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
  - Clean up meson.build (remove foreach loop, replace with single call)
- Update SConscript
  - use `$SOURCES` to call `CodeGenerate` with multiple source files

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>