cmd/compile: add testing-flag guard to package-is-collected assert

On advice of the department of garbage collection, forcing a garbage
collection generally does not improve performance. However,
this-data-is-now-unreachable is a good property to be able to test,
and that requires finalizers and a forced GC. So, to save build time,
this test was removed from the compiler itself, but to verify the
property, it was added to the fma_test (and the end-to-end dependence
on the flag was tested with an inserted failure in testing the
test).

TODO: also turn on the new -d=gccheck=1 debug flag on the ssacheck
builder.

Benchmarking reveals that it is profitable to avoid this GC,
with about 1.5% reduction in both user and wall time.

(48 p) https://perf.golang.org/search?q=upload:20221103.3
(12 p) https://perf.golang.org/search?q=upload:20221103.5

Change-Id: I4c4816d619735838a32388acf0cc5eb1cd5f0db5
Reviewed-on: https://go-review.googlesource.com/c/go/+/447359
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/base/debug.go

@@ -27,6 +27,7 @@ type DebugFlags struct {
 	Export                int    `help:"print export data"`
 	Fmahash               string `help:"hash value for use in debugging platform-dependent multiply-add use" concurrent:"ok"`
 	GCAdjust              int    `help:"log adjustments to GOGC" concurrent:"ok"`
+	GCCheck               int    `help:"check heap/gc use by compiler" concurrent:"ok"`
 	GCProg                int    `help:"print dump of GC programs"`
 	Gossahash             string `help:"hash value for use in debugging the compiler"`
 	InlFuncsWithClosures  int    `help:"allow functions with closures to be inlined" concurrent:"ok"`