libc: tests: hook CPUSET(9) test up to the build

Add shims to map NetBSD's API to CPUSET(9). Obviously the invalid input
parts of these tests are relatively useless since we're just testing the
shims that aren't used elsewhere, there's still some amount of value in
the parts testing valid inputs.

Differential Revision: https://reviews.freebsd.org/D27307

(cherry picked from commit 9e1281eabafa4aaf84828e70488c1802717b59af)