posix: sysconf: match _SC* defines with newlib/picolib

It's possible for newlib/picolib libc libraries to
internally call sysconf() which would execute zephyr's
implementation.  However, if the _SC* defines do not have
matching values, then the incorrect switch case executes.

This issue arises when using newlib/picolib libc that includes
sysconf implementation for ARM.  With current defaults, the
zephyr sysconf() overrides the original libc sysconf() so
we must ensure proper operation.

For Full Sysconf implementation, we will switch to the #define
list just like newlib/picolib.
We can't currently use their unistd.h directly due to a domino
of declaration conflicts.
For the "small" macro implementation, it remains with an
enum list which is required for the macro to work.

Signed-off-by: Nicholas Lowell <[email protected]>

Author: Nicholas Lowell