Raspberry Pi: Make it compile OOB on AArch64

[egrath]

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Behavior: Does not compile out of the box on Raspberry Pi OS 64-bit because AArch64 isn't handled correctly in the build system

How to reproduce: Install Raspberry Pi Os 64-bit and follow the build instructions, it will fail with some unknown CPU error

Workaround: Tweak the file shared/h/shared/system.h by replacing:

#elif (defined __arm__)

with

#elif (defined __arm__ || defined __aarch64__)

[tom-seddon]

Excellent, thank you. I'll make this change.

[scruss]

Can confirm that git clone --recursive --branch 'wip/master' https://github.com/tom-seddon/b2.git then the usual build process produces a working b2 setup on a 64-bit Raspberry Pi 5