linux/compat.h: port lower_32_bits and upper_32_bits from Linux

[backport from linux commit 204b885e and 218e180e7]
64 bit processors are becomming more and more popular.
lower_32_bits and upper_32_bits save our labor doing
shifts/manipulations like (u32)(n) and (u32)((n) >> 32).
They are good helpers in both little and big endian cases.
Port these two functions here from Linux:include/linux/kernel.h,
cater the comment message to little/big endian cases.
Later on, developers could include linux/compat.h if they want to
use these two functions.

Signed-off-by: Lijun Pan <Lijun.Pan@freescale.com>