Partial port of the ARM Neon for aarch64.

Benchmarks from fate:

benchmarking with Linux Perf Monitoring API
nop: 58.6
checkasm: using random seed 1760970128
NEON:
 - vp8dsp.idct       [OK]
 - vp8dsp.mc         [OK]
 - vp8dsp.loopfilter [OK]
checkasm: all 21 tests passed
vp8_idct_add_c: 201.6
vp8_idct_add_neon: 83.1
vp8_idct_dc_add_c: 107.6
vp8_idct_dc_add_neon: 33.8
vp8_idct_dc_add4y_c: 426.4
vp8_idct_dc_add4y_neon: 59.4
vp8_loop_filter8uv_h_c: 688.1
vp8_loop_filter8uv_h_neon: 216.3
vp8_loop_filter8uv_inner_h_c: 649.3
vp8_loop_filter8uv_inner_h_neon: 195.3
vp8_loop_filter8uv_inner_v_c: 544.8
vp8_loop_filter8uv_inner_v_neon: 131.3
vp8_loop_filter8uv_v_c: 706.1
vp8_loop_filter8uv_v_neon: 141.1
vp8_loop_filter16y_h_c: 668.8
vp8_loop_filter16y_h_neon: 242.8
vp8_loop_filter16y_inner_h_c: 647.3
vp8_loop_filter16y_inner_h_neon: 224.6
vp8_loop_filter16y_inner_v_c: 647.8
vp8_loop_filter16y_inner_v_neon: 128.8
vp8_loop_filter16y_v_c: 721.8
vp8_loop_filter16y_v_neon: 154.3
vp8_loop_filter_simple_h_c: 387.8
vp8_loop_filter_simple_h_neon: 187.6
vp8_loop_filter_simple_v_c: 384.1
vp8_loop_filter_simple_v_neon: 78.6
vp8_put_epel8_h4v4_c: 3971.1
vp8_put_epel8_h4v4_neon: 855.1
vp8_put_epel8_h4v6_c: 5060.1
vp8_put_epel8_h4v6_neon: 989.6
vp8_put_epel8_h6v4_c: 4320.8
vp8_put_epel8_h6v4_neon: 1007.3
vp8_put_epel8_h6v6_c: 5449.3
vp8_put_epel8_h6v6_neon: 1158.1
vp8_put_epel16_h6_c: 6683.8
vp8_put_epel16_h6_neon: 831.8
vp8_put_epel16_h6v6_c: 11110.8
vp8_put_epel16_h6v6_neon: 2214.8
vp8_put_epel16_v6_c: 7024.8
vp8_put_epel16_v6_neon: 799.6
vp8_put_pixels8_c: 112.8
vp8_put_pixels8_neon: 78.1
vp8_put_pixels16_c: 131.3
vp8_put_pixels16_neon: 129.8

This contains a fix to include guards by Carl Eugen Hoyos.
Signed-off-by: Martin Storsjö <martin@martin.st>