Blackfin: kgdb_test: rework code to avoid -O0 usage

__kfree_rcu() in rcupdate.h bugs when parameter offset is not a constant
at compile time.  Since we build the kgdb_test module with -O0 and it
includes this header file, we hit the bug.  So drop the -O0 and mark the
one func we need for the test as noinline (so we can set a breakpoint on
it and have it be hit).

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>