This fixes the following build error, spotted by the LEDE buildbots:

{standard input}: Assembler messages:
{standard input}:557: Error: operand 3 should be an integer register -- `mul x0,x0,1048576'
{standard input}:558: Error: operand 3 should be an integer register -- `smulh x1,x0,1048576'
Makefile:1466: recipe for target 'ext/opcache/zend_accelerator_module.lo' failed

Signed-off-by: Michael Heimpold <mhei@heimpold.de>