runtime: call mallocgc directly from makeslice and growslice

runtime: call mallocgc directly from makeslice and growslice

The extra checks provided by newarray are
redundant in these cases.

This shrinks by one frame the call stack expected
by the pprof test.

name                      old time/op  new time/op  delta
MakeSlice-8               34.3ns ± 2%  30.5ns ± 3%  -11.03%  (p=0.000 n=24+22)
GrowSlicePtr-8             134ns ± 2%   129ns ± 3%   -3.25%  (p=0.000 n=25+24)

Change-Id: Icd828655906b921c732701fd9d61da3fa217b0af
Reviewed-on: https://go-review.googlesource.com/22276
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>