libsysdecode: Add a new ABI type, SYSDECODE_ABI_CLOUDABI32.

In order to let truss(8) support tracing of 32-bit CloudABI
applications, we need to add a new ABI type to libsysdecode. We can
reuse the existing errno mapping table. Also link in the cloudabi32
system call table to translate system call names.

While there, remove all of the architecture ifdefs. There are not
needed, as the CloudABI data types and system call tables build fine on
any architecture. Building this unconditionally will make it easier to
do tracing for different compat modes, emulation, etc.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D13516