[freebsd] Fixing compilation for FreeBSD/amd64 using gcc

[erikarn]

System information

Type	Version/Name
Distribution Name	FreeBSD

Describe the problem you're observing

There are a couple of issues compiling ZFS on FreeBSD with GCC. I'm testing with 6.4 and 9.x; I'll update to GCC-10 once I wrangle the previous versions to work right.

• The FreeBSD ioctl API assumes pointers are 64 bits, which isn't true on 32 bit platforms or when compiling the 32 bit compat library
• The FreeBSD print argument checking looks copy/paste'd from illumnos and assumes the existence of cmn_err as a format attribute. This path is only run when compiling with GCC; it doesn't show up in LLVM.

Describe how to reproduce the problem

• compile freebsd-head + ZFS using GCC on amd64

Include any warning/errors/backtraces from the system logs