Refactoring: move out generic bits from cloudabi64_sysvec.c.

In order to make it easier to support CloudABI on ARM64, move out all of
the bits from the AMD64 cloudabi_sysvec.c into a new file
cloudabi_module.c that would otherwise remain identical. This reduces
the AMD64 specific code to just ~160 lines.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D3974