atomic-long: Fix warnings on arm64

Several inline functions in this file reference undefined functions in
U-Boot.  For example:

atomic-long.h:73:9: warning: implicit declaration of function
'atomic64_sub_and_test'
atomic-long.h:80:9: warning: implicit declaration of function
'atomic64_dec_and_test'
atomic-long.h:87:9: warning: implicit declaration of function
'atomic64_inc_and_test'

Handle this the same as the 32 bit build by wrapping these functions in
a __UBOOT__ check.

Signed-off-by: Bradley Bolen <bradleybolen@gmail.com>