cmd/compile: don't use fast32/64 map functions for aggregates

Under register ABI, aggregates like [2]uint32 is passed
differently than a uint64. For now, don't use the fast version
of the map functions for non-trivial aggregates.

GOEXPERIMENT=regabi,regabiargs can now pass make.bash, modulo
staleness checks.

TODO: maybe find some way to use the fast functions. Maybe
unsafe-cast to uint32/64 then call the map function. But need to
make the type checker happy.

Change-Id: If42717280dde12636fb970798cf1ca8fb29a3d06
Reviewed-on: https://go-review.googlesource.com/c/go/+/308650


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>