Easier access to compiler endianness

[IcedQuinn]

Access to whether the compiler is on a big or little endian system is useful for some bit operations.

For example in the xxhash algorithm in Nim:

proc XXH_readLE32(p: pointer): uint32 =
    when cpu_endian == little_endian:
        return XXH_read32(p)
    else:
        return XXH_swap32(XXH_read32(p))

I searched for endian in the documentation search engine and with silver searcher in the codebase. It looks like cdefs.lua does mention some endian checking but the docs don't mention it.

[edubart]

You can check for this like in the example:

## if ccinfo.is_little_endian then
  print 'little endian'
## elseif ccinfo.is_big_endian then
  print 'big endian'
## else
  print 'endianess is unknown'
## end