selinux: hweight optimization in avtab_read_item

avtab_read_item() is a hot function called when reading each rule in a
binary policydb. With the current Fedora policy and refpolicy, this
function is called nearly 100,000 times per policy load.

A single avtab node is only permitted to have a single specifier to
describe the data it holds. As such, a check is performed to make sure
only one specifier is set. Previously this was done via a for-loop.
However, there is already an optimal function for finding the number of
bits set (hamming weight) and on some architectures, dedicated
instructions (popcount) which can be executed much more efficiently.

Even when using -mcpu=generic on a x86-64 Fedora 38 VM, this commit
results in a modest 2-4% speedup for policy loading due to a substantial
reduction in the number of instructions executed.

Signed-off-by: Jacob Satterfield <[email protected]>
Reviewed-by: Stephen Smalley <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

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