checkasm: add cycle counter read for aarch64

Needs kernel support since user space access to the cycle counter is not
allowed on all available AArch64 systems (Android 5 and iOS).
aarch64: cabac_encode_{decision,bypass,terminal}_asm

benchmarks on a Nexus 9 (nvidia denver):
101.3 cycles in x264_cabac_encode_decision_c,   67105369 runs, 3495 skips
 97.3 cycles in x264_cabac_encode_decision_asm, 67105493 runs, 3371 skips
132.8 cycles in x264_cabac_encode_terminal_c,    1046950 runs, 1626 skips
116.1 cycles in x264_cabac_encode_terminal_asm,  1048424 runs, 152 skips
 92.4 cycles in x264_cabac_encode_bypass_c,     16776192 runs, 1024 skips
 89.6 cycles in x264_cabac_encode_bypass_asm,   16776453 runs, 763 skips

Cycle counts are not as stable as one would like. The dynamic code
optimisation seems to produce different results for small chnages in a
binary. Repeated runs with the same binary produce stable results
though (ignoring the first run).
NV21 input support

Eliminates an extra copy when encoding Android camera preview images.

Checkasm test by Janne Grunau.
ARM assembly with improvements from Janne Grunau.
x86: SSE and AVX implementations of plane_copy

Remove the MMX2 implementation.
Remove explicit prefetches (we're reading data sequentially so automatic prefetching handles that just fine).