posix: sysconf: use Z_SC_VAL_IFDEF for sysconf constants

Previously, CONFIG_MAX_TIMER_COUNT and CONFIG_MSG_COUNT_MAX were
used without checking if CONFIG_TIMER and CONFIG_POSIX_MQUEUE
were enabled.

Use Z_SC_VAL_IFDEF() to conditionally define those constants
based on their gating Kconfig values, otherwise, they are defined
to -1.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>