s390x: Add test_bit to library

Query/feature bits are commonly tested via MSB bit numbers on
s390. Let's add test bit functions, so we don't need to copy code to
test query bits.

The test_bit code has been taken from the kernel since most s390x KVM unit
test developers are used to them.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>