Fix buffer alignment issues on ESP32 and ARM platforms

[Bwooce]

Summary

This PR fixes buffer alignment issues that cause MAVLink message corruption on ESP32 (Xtensa) and ARM platforms.

Problem

ESP32 and ARM processors require proper memory alignment when accessing multi-byte values (float, int32, etc.) through char buffers. Without proper alignment, these platforms can experience:

• Data corruption when reading/writing values
• Stack corruption leading to incorrect message lengths
• ATTITUDE messages reporting 385 bytes instead of 28 bytes

Solution

• Added MAVLINK_ALIGNED_BUF macro that uses alignas(4) for ESP32/ARM platforms
• Updated code generator to use MAVLINK_ALIGNED_BUF instead of plain char[] arrays
• Included stdalign.h when alignment is needed

Changes

1. generator/C/include_v2.0/protocol.h: Added MAVLINK_ALIGNED_BUF macro definition
2. generator/mavgen_c.py: Updated all buffer declarations to use the new macro

Testing

• Tested on ESP32-S3 hardware where the issue was originally observed
• Verified MAVLink messages are now correctly sized and uncorrupted
• No impact on other platforms (macro defaults to plain char[] on non-ARM/ESP32)

Related Issues

This resolves MAVLink corruption issues observed in ArduPilot on ESP32 platforms.

[tridge]

@Bwooce thanks for raising this - what is surprising is arm (specifically STM32) is the most widely used platform that MAVLink is used on. So why are we not seeing issues in current usage? I think that for MAVLINK_NEED_BYTE_SWAP or !MAVLINK_ALIGNED_FIELDS it should be mapping the _mav_put_XXX() functions onto byte_copy_YY() which maps to a function that copies byte by byte. We could use what you've done and then potentially make things a bit more efficient by avoiding the byte by byte copies