x/build: add longtest builder for linux-arm

[cagedmantis]

Forking this from #49649

According to https://golang.org/wiki/PortingPolicy#first-class-ports, linux/arm is currently a “first class ports”.

However, we have no longtest builder coverage whatsoever for this architecture. We ought to have builders running the tests at the same level of depth as linux-386, linux-amd64 and linux-arm64.

cc @golang/release @bcmills @cherrymui

[aclements]

Just cross-referencing some info from #49649. This is probably blocked on #56693 since we don't have the capacity in the current linux-arm builders to scale up to a longtest builder.