Add macro for unaligned access.

[HiFiPhile]

Describe the PR
A proposition for unaligned access macro.

Then we can refactor unaligned accesses, for example:

uint16_t total_len;
// Use offsetof to avoid pointer to the odd/misaligned address
memcpy(&total_len, (uint8_t*) desc_bos + offsetof(tusb_desc_bos_t, wTotalLength), 2);

Into

uint16_t total_len;
TU_UNALIGNED_GET(total_len, &desc_bos->wTotalLength, uint16_t);

Additional context
It should works on TI compiler, need test.

[hathach]

thank you for your PR, it looks good. However, I think this shouldn't be part of the compiler. I am thinking to have an typdef for tu_unaligned_uint32_t then using static inline function for the assignment. I will make an PR to demonstrate the idea.

UPDATE: ah I see, zephyr use macro so that it can be opt-out when using with MCU that can do unaligned access like m3, m4

[hathach]

closed in favor of #772