linux: Use sched_getaffinity for __get_nprocs (BZ #27645)

Both the sysfs and procfs parsing (through GET_NPROCS_PARSER) are
removed in favor the syscall.  The initial scratch buffer should
fit to most of the common usage (1024 bytes with maps to 8192 CPUs).

Checked on x86_64-linux-gnu and aarch64-linux-gnu.