1. Add "av1_" prefix for public functions
av1_build_compound_diffwtd_mask_c and
av1_build_compound_diffwtd_mask_highbd_c

2. Move them to rtcd, SIMD version will be added in
following up CL.

3. Remove sb_type from argument list, given that
block_size_wide[sb_type] is always equal to the width.

Change-Id: I6a7be770b8acd040c2f167d7aca08cb9e7b3a78e