`armv7a-linux` -> `arm-linux`

[alexrp]

FYI: ziglang/www.ziglang.org@75bee0d

This will take effect in master builds ~immediately, and release builds from 0.15.0.

[kassane]

This cause (tag: v2025_05_24) in docker-cross arm/v7-image:

1.745 mkdir -p '/zig'
3.521 thread 25 panic: attempt to use null value
3.522 Unable to dump stack trace: debug info stripped
4.364 Aborted (core dumped)

cc: @marler8997