tests: kernel/common: avoid using compiler builtin popcount

Not all arch has native support for __builtin_popcount() on
hardware and GCC falls back in using software only implementation.
However, with GCC 11, this is no longer included automatically
and requires linking explicitly with libgcc.a. This is not
trivial as it requires changes some linker magic and a sizable
change to most linker scripts. So opt for an easy solution
by implementing our own popcount in the test.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>