aom_dsp: remove x86inc.asm distinction

Manually cherrypicked from libvpx/master:
1b833d63d9c82270e4ea588541d14e9111c64c79
a4f3751be5f012d66011ddc1c5f12bd12734a1d3

Change-Id: I3b82e54a3173ac1458a13f33fd36094fec066f1c
8 files changed