cmd/compile: grow stack before test() to avoid gdb misbehavior

While next-ing over a call in gdb, if execution of that call
causes a goroutine's stack to grow (i.e., be moved), gdb loses
track and runs ahead to the next breakpoint, or to the end of
the program, whichever comes first.

Prevent this by preemptively growing the stack so that
ssa/debug_test.go will reliably measure what is intended,
the goodness of line number placement and variable printing.

Fixes #25497.

Change-Id: I8daf931650292a8c8faad2285d7fd405f2157bd2
Reviewed-on: https://go-review.googlesource.com/114080


Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>