Golang x/build: update amd64 builder instance types

We have cryptography assembly not tested in CI for lack of amd64 SHA extensions (#69592) and more recently a potential AVX-512 failure which doesn't have CI coverage (#74767).

The C3D or C4D GCE instance types run on AMD Zen 4+, which offer SHA, AVX-512, and generally anything we might want.

The C3 or C4 instance types run Intel Sapphire Rapids, which should offer SHA and AVX-512, but maybe not SHA-512 (which we don't have assembly for yet), but reading Intel compatibility tables makes my eyes bleed so I am not sure, to be honest.

Can we update the amd64 builder instance types (or, alternatively, spin up a new longtest and maybe TryBot)?

/cc @golang/release