Add armv7l-linux-gnueabihf builder, tester and uploader

[staticfloat]

Because testing on armv7l is painful, we run only a very small set of
tests, pending greater investment in armv7l infrastructure.

[staticfloat]

This is going to sit and wait for us to actually have armv7l builders again. I've signed up for backorder notifications for 8GB rpi4's, which should hopefully be sufficient for our build needs.

[inkydragon]

Maybe we could get some free ARM computing resources with funding provided by Arm Inc.
(miniNodes/Open Source Lab/Equinix)
https://www.arm.com/solutions/infrastructure/works-on-arm

[staticfloat]

And after many moons of inactivity, we have finally procured armv7l hardware!

Now begins the great debugging spree:

error: 'bt_context_t' {aka 'struct unw_tdep_context'} has no member named 'uc_mcontext'

[giordano]

JuliaLang/julia#47345

[topolarity]

Would it be easier to manage the workers if we used Aarch64 machines running in 32-bit mode (assuming that we have those available)?

I did something like that recently to test out some ARMv7 changes, and it was surprisingly smooth with multi-arch on Debian.

[giordano]

I'm not entirely sure, but I seem to remember JuliaLang/julia#41064 couldn't be reproduced on aarch64 running the armv7l binary.

[topolarity]

Hmm - some coverage is better than no coverage?

Oof, building on aarch64 in an armhf docker container shows plenty of issues for me