Rename armv7a-linux to arm-linux

[alexrp]

This is the only outlier among the tarballs where the tarball's arch name doesn't match the triple's arch name exactly. ARMv7 has been our baseline for a long time, and it's extremely unlikely that anyone is trying to run the Zig compiler on ARMv6 (or older), so I don't think there'd be any confusion as a result of this change.