introduce generic posix_types.h ...

Most types defined in posix_types.h are architecture independent,
so instead of repeating this for each architecture add a generic
file that can be used by architecture code. To use it an architecture
must define BITS_PER_LONG properly in asm/bitsperlong.h.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>