runtime/debug: don't trigger a GC on SetGCPercent
    
    Currently SetGCPercent forces a GC in order to recompute GC pacing.
    Since we can now recompute pacing on the fly using gcSetTriggerRatio,
    change SetGCPercent (really runtime.setGCPercent) to go through
    gcSetTriggerRatio and not trigger a GC.
    
    Fixes #19076.
    
    Change-Id: Ib30d7ab1bb3b55219535b9f238108f3d45a1b522
    Reviewed-on: https://go-review.googlesource.com/39835
    Run-TryBot: Austin Clements <austin@google.com>
    Reviewed-by: Rick Hudson <rlh@golang.org>