leveldb: Add ARMv8 CRC32C support #12968

Use the optional CRC32 instructions to accelerate leveldb's CRC32C computation on ARMv8 (32 and 64 bit). Uses a similar plan as the SSE42 instructions for x86.

Needs testing. What I've tested:

• Build system detection seems to work; only if the compiler supports the instructions and HWCAPx_CRC32 exists in the kernel headers, the specific compilation unit is built
• Instruction set is successfully detected and used on Odroid-C2 AARCH64, CRC computations are correct
• Instruction set is (correctly) not detected, and not used on Cubox/i.MX6 ARM32.

Fails travis test only because: the new src/leveldb/* files should go to https://github.com/bitcoin-core/leveldb, most of the changes here are build system, though.

Benchmarking just the CRC instructions vs a software implementation of CRC32C, I've found that usually it's about 6 times faster (if you want to try on your own platform see my crcbench):

• Amlogic ARM Cortex-A53 1.5Ghz (Odroid-C2)

[sw] 65543000 bytes in 139538us monotonic 138962us CPU (471.7MB/s) 3.3 cycles/b
[hw-arm] 65543000 bytes in 22863us monotonic 22838us CPU (2869.9MB/s) 0.5 cycles/b

• HiSilicon 620 processor: 8 ARM Cortex A53 cores, 1.2 ghz (Hikey)

[sw] 65543000 bytes in 159145us monotonic 158469us CPU (413.6MB/s) 2.8 cycles/b
[hw-arm] 65543000 bytes in 29079us monotonic 28981us CPU (2261.6MB/s) 0.5 cycles/b

Will add i.MX8 results later.

Was apparently already done here: https://github.com/bitcoin-core/leveldb/pull/17/commits/5c39524f3639e6bf6ab49215152d24273e662986