FreeBSD libzfs: gcc requires __thread after static

Building libzfs with gcc on FreeBSD failed because gcc is picky about
the order of keywords in declarations with __thread, whereas clang is
more relaxed.

https://gcc.gnu.org/onlinedocs/gcc/Thread-Local.html

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Closes #11331