Support uClibc for the tests compilations

There are two issues that don't allow ZFS to be compiled using uClibc.
backtrace(), and program_invocation_short_name as a const.
This patch adds uClibc to the conditionals in the same way there are
already for Glibc for backtrace(); and removes the external param
program_invocation_short_name because its only used here for the
whole project.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Closes #11600