mempool/octeontx2: fix clang build for arm64

mempool/octeontx2: fix clang build for arm64

The ARMv8.1 CASP instruction works with even register pairs and since
there no register constraint in older versions of GCC/Clang, use
explicit register allocation to satisfy CASP requirements.
Remove function level optimization specification as Clang doesn't have
support for it and explicit register allocation removes the need for it.

Fixes build issue with arm64-armv8a-linux-clang.

Fixes: ee338015e7a9 ("mempool/octeontx2: add optimized dequeue operation for arm64")

Reported-by: Gavin Hu <gavin.hu@arm.com>
Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>