net: make __net_buf_align 64-bit compatible
net: make __net_buf_align 64-bit compatible

Structures to which this applies contain pointers. So the alignment
should depend on pointer width. On 32-bit builds this remains the same.

Signed-off-by: Nicolas Pitre <76342682d5afc62854e1102d9578521fe1ed6813@baylibre.com>

GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr