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.

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, we have to drop using
CONCAT to prevent pre-expansion of the new #defines

Signed-off-by: Nicholas Lowell <Nicholas.Lowell@lexmark.com>