libc: minimal: Fix gmtime() userspace support

The gmtime() function returns a global result variable, and this
variable must be placed in the `z_libc_partition` when userspace is
enabled.

Since gmtime() makes use of a global variable and this results in a
footprint increase, this commit makes the time functions optional by
introducing `CONFIG_MINIMAL_LIBC_TIME` Kconfig and making them only
available when this option is enabled.

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