x86: Add 6-tap variants of high bit-depth mc SSSE3 functions

Checkasm numbers on Zen 4 (64-bit):

            8-tap    6-tap
w2_v:        22.0     19.2
w2_hv:       37.9     34.6

w4_v:        29.2     25.1
w4_hv:       80.6     48.3

w8_h:        57.5     47.5
w8_v:        53.3     40.8
w8_hv:      157.1    101.5

w16_h:      158.4    129.3
w16_v:      140.5    104.5
w16_hv:     413.9    263.2

w32_h:      497.6    405.5
w32_v:      431.1    318.5
w32_hv:    1246.4    792.1

w64_h:     1766.7   1433.2
w64_v:     1564.6   1159.5
w64_hv:    4273.5   2744.6

w128_h:    4989.1   4047.5
w128_v:    4483.4   3399.3
w128_hv:  11962.4   7727.9