feat(build): add Raspberry Pi 5 (Cortex-A76) support

[sbs44]

Summary

Add support for building Flutter engine binaries optimized for Raspberry Pi 5 (Cortex-A76).

Changes

• Add pi5 to CPU enum with Cortex-A76 compiler flags
• Add pi5_64 target for 64-bit ARM engine builds
• Update README with Pi5 build configuration and compiler invocation table

Compiler flags

arm_cpu = "cortex-a76+nocrypto"
arm_tune = "cortex-a76"

Resulting in: --target=aarch64-linux-gnu -mcpu=cortex-a76+nocrypto -mtune=cortex-a76

Notes

• Pi5 is 64-bit only (no 32-bit variant)
• Using +nocrypto for consistency with Pi3/Pi4 builds, though the A76 in Pi5 does support crypto extensions

Test plan

• Workflow builds pi5-64 profile and release engines successfully
• Built and deployed Flutter app to Raspberry Pi 5 hardware
• Verified app runs correctly with Pi5-optimized engine

[sbs44]

Related: ardera/flutterpi_tool#81 adds the --cpu=pi5 CLI flag that uses these engine binaries.

[ardera]

AFAICT the Pi 5 actually supports crypto extensions¹, so we can remove the +nocrypto in the arm_cpu.

Rest looks good! Thanks for the contribution!

Footnotes

1. https://pip-assets.raspberrypi.com/categories/892-raspberry-pi-5/documents/RP-008348-DS-4-raspberry-pi-5-product-brief.pdf ↩

[sbs44]

Thanks for the review and for pointing that out! I've updated the code to remove +nocrypto from the Pi5 CPU flags since the Cortex-A76 supports crypto extensions.

[ardera]

The failures are due to the CI running on a fork, AFAICT, the rest seems to work, so I'll bypass the checks and merge