Fixes: https://github.com/openwrt/packages/issues/16271

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Initial draft PR is:
  https://github.com/openwrt/packages/pull/11894

This one is a bit more complete, and follows packaging practices.

For now, disabling builds on ARC and PowerPC. Will require more work to get
them going.
Explicitly disabling OpenMP support, so that it doesn't get picked by
accident.

Later we may use the `CPU_TYPE` parameter to tweak things a little further.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>