Use ARMv8 CRC instructions where available.

 Use ARMv8 CRC instructions where available.
 
 ARMv8 introduced special CPU instructions for calculating CRC-32C. Use
 them, when available, for speed.
 
 Like with the similar Intel CRC instructions, several factors affect
 whether the instructions can be used. The compiler intrinsics for them must
 be supported by the compiler, and the instructions must be supported by the
 target architecture. If the compilation target architecture does not
 support the instructions, but adding "-march=armv8-a+crc" makes them
 available, then we compile the code with a runtime check to determine if
 the host we're running on supports them or not.
 
 For the runtime check, use glibc getauxval() function. Unfortunately,
 that's not very portable, but I couldn't find any more portable way to do
 it. If getauxval() is not available, the CRC instructions will still be
 used if the target architecture supports them without any additional
 compiler flags, but the runtime check will not be available.
 
 Original patch by Yuqi Gu, heavily modified by me. Reviewed by Andres
 Freund, Thomas Munro.
 
 Discussion: https://www.postgresql.org/message-id/HE1PR0801MB1323D171938EABC04FFE7FA9E3110%40HE1PR0801MB1323.eurprd08.prod.outlook.com