Improve mc-a.S Performance by Using SVE/SVE2

Imporve the performance of NEON functions of aarch64/mc-a.S
by using the SVE/SVE2 instruction set. Below, the specific functions
are listed together with the improved performance results.

Command executed: ./checkasm8 --bench=avg
Testbed: Alibaba g8y instance based on Yitian 710 CPU
Results:
avg_4x2_c: 274
avg_4x2_neon: 215
avg_4x2_sve: 171
avg_4x4_c: 461
avg_4x4_neon: 343
avg_4x4_sve: 225
avg_4x8_c: 806
avg_4x8_neon: 619
avg_4x8_sve: 334
avg_4x16_c: 1523
avg_4x16_neon: 1168
avg_4x16_sve: 558

Command executed: ./checkasm8 --bench=avg
Testbed: AWS Graviton3
Results:
avg_4x2_c: 267
avg_4x2_neon: 213
avg_4x2_sve: 167
avg_4x4_c: 467
avg_4x4_neon: 350
avg_4x4_sve: 221
avg_4x8_c: 784
avg_4x8_neon: 624
avg_4x8_sve: 302
avg_4x16_c: 1445
avg_4x16_neon: 1182
avg_4x16_sve: 485