build system: Make it possible to build with 64bit time_t

On most 32bit architectures time_t (and a few other time related types)
are a signed 32bit wide integer type.
As a consequence they can only represent dates between

	Fri Dec 13 08:45:52 PM UTC 1901

(-0x80000000 seconds before Jan 1 1970) and

	Tue Jan 19 03:14:07 AM UTC 2038

(0x7fffffff seconds after Jan 1 1970). Given that some machines that are
built today have an expected lifetime of >15 years, this needs to be
extended. To to that, define the cpp symbol _TIME_BITS to 64 which
results in some magic in glibc to make time_t (and the few other time
related types) 64 bit wide.

This new switch CONFIG_TIME64 is in the spirit of CONFIG_LFS and only
expected to have the expected effect with glibc. On musl for examples
time_t already defaults to 64bit wide types.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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