aarch64: mpi/longlong.h: fix operand size mismatch

* mpi/longlong.h [__aarch64__] (count_leading_zeros): Use correctly
sized temporary variable for asm output.

Patch fixes clang-8 warning about differently sized inline
assembly operands seen on aarch64.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>