cmd/link: establish dependable package initialization order

(This is a retry of CL 462035 which was reverted at 474976.
The only change from that CL is the aix fix SRODATA->SNOPTRDATA
at inittask.go:141)

As described here:

https://github.com/golang/go/issues/31636#issuecomment-493271830

"Find the lexically earliest package that is not initialized yet,
but has had all its dependencies initialized, initialize that package,
 and repeat."

Simplify the runtime a bit, by just computing the ordering required
in the linker and giving a list to the runtime.

Update #31636
Fixes #57411

RELNOTE=yes

Change-Id: I28c09451d6aa677d7394c179d23c2c02c503fc56
Reviewed-on: https://go-review.googlesource.com/c/go/+/478916
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -119,6 +119,10 @@ type ArchSyms struct {
 	DynStr  loader.Sym
 
 	unreachableMethod loader.Sym
+
+	// Symbol containing a list of all the inittasks that need
+	// to be run at startup.
+	mainInittasks loader.Sym
 }
 
 // mkArchSym is a helper for setArchSyms, to set up a special symbol.