drivers rtc: Do not treat the host libC differently

This code was using strptime from the C library in some cases,
but this function is an extension which many embedded libCs do
not provide, and which is not provided by default unless
_XOPEN_SOURCE or a similar macro is defined before the system headers
are included.

We could define _XOPEN_SOURCE for the libraries that provide it,
but instead of treating the host C library differently than
embedded libraries, let's just build the provided version always,
as that should provide better coverage of this code.

Note: It seems picolibc's strptime is broken,
until this very recent patch:
https://github.com/picolibc/picolibc/pull/657


so we should not select it when building for this library
for a while either.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>