Provide CPU capability bits for Linux

Some toolchains might be missing CPU capability in <bits/hwcap.h> so
alternative definition needed to make sure library would compile
properly in a such case.

Signed-off-by: Anton Bondarenko <anton.bondarenko@arm.com>

Closes #2

Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>