[dev.regabi] cmd/compile: fix GOEXPERIMENT=regabi builder

I misread the FIXME comment in InitLSym the first time. It's referring
to how InitLSym is supposed to be called exactly once per
function (see function documentation), but this is evidently not
actually the case currently in GOEXPERIMENT=regabi mode.

So just move the NeedFuncSym call below the GOEXPERIMENT=regabi
workaround.

Also, to fix the linux-arm64-{aws,packet} builders, move the call to
reflectdata.WriteFuncSyms() to after the second batch of functions are
compiled. This is necessary to make sure we catch all the funcsyms
that can be added by late function compilation.

Change-Id: I6d6396d48e2ee29c1fb007fa2b99e065b36375db
Reviewed-on: https://go-review.googlesource.com/c/go/+/283552


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>