libc: minimal: Introduce CONFIG_MINIMAL_LIBC_NON_REENTRANT_FUNCTIONS

This commit introduces a new configuration called
`CONFIG_MINIMAL_LIBC_NON_REENTRANT_FUNCTIONS`, which enables the
traditional non-reentrant (i.e. not thread-safe) version of the C
standard library functions such as rand() and gmtime() when the
respective configs are enabled.

The non-reentrant functions make use of the globals and require an
additional memory partition (MPU region), which is scarce on low-end
devices, when CONFIG_USERSPACE=y.

The purpose of this option is to classify the MPU resource intensive
functions as a separate category and only enable them when there is a
demand for such.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>