Implement reading CPU count

To make uutils' nproc working, by reading /scheme/sys/cpu.

It returns the correct value (both in QEMU and real device tested):

ARG_MAX (0): 4096 (0)
CHILD_MAX (1): 65536 (0)
CLK_TCK (2): 100 (0)
NGROUPS_MAX (3): 65536 (0)
OPEN_MAX (4): 1024 (0)
STREAM_MAX (5): 16 (0)
TZNAME_MAX (6): -1 (0)
VERSION (29): 200809 (0)
PAGESIZE (30): 4096 (0)
RE_DUP_MAX (44): 32767 (0)
LOGIN_NAME_MAX (71): 256 (0)
TTY_NAME_MAX (72): 32 (0)
SYMLOOP_MAX (173): -1 (0)
HOST_NAME_MAX (180): 64 (0)
NPROCESSORS_CONF (57): 4 (0)
NPROCESSORS_ONLN (58): 4 (0)

EDIT: I think uutils nproc read from std https://github.com/rust-lang/rust/blob/ddaf12390d3ffb7d5ba74491a48f3cd528e5d777/library/std/src/sys/thread/unix.rs#L327-L330