[microTVM][Zephyr][AOT] Memory Alignment

[mehrdadh]

There's an issue with AOT demo where the zephyr RTOS doesn't guarantee that g_aot_memory start with a 4 bytes alignment memory address. Therefore, it will cause an exception specially on fp32 operations. One simple hack is to add memory alignment to g_aot_memory which is used by stack allocator:

static uint8_t __attribute__((aligned(4))) g_aot_memory[WORKSPACE_SIZE];

We need to investigate more to figure out whether this is a Zephyr RTOS bug or there's a configuration that is missing in our Zephyr AOT demo. Also our current test_zephyr_aot.py doesn't catch this problem. Ideally we should add a test to check this specifically for Zephyr RTOS.

Thanks to @areusch and @gromero for helping to find this issue.

[mehrdadh]

PR is merged now!