The Go compiler assumes that pointers escape when passed into assembly
functions. To override this behavior we can annotate assembly functions
with go:noescape, telling the compiler that we know pointers do not
escape from it.

By annotating the assembly functions in the s390x P256 code in this way
we enable more variables to be allocated on the stack rather than
the heap, reducing the number of heap allocations required to execute
this code:

name        old alloc/op   new alloc/op   delta
SignP256      3.66kB ± 0%    2.64kB ± 0%  -27.95%  (p=0.008 n=5+5)
VerifyP256    4.46kB ± 0%    1.23kB ± 0%  -72.40%  (p=0.008 n=5+5)

name        old allocs/op  new allocs/op  delta
SignP256        40.0 ± 0%      31.0 ± 0%  -22.50%  (p=0.008 n=5+5)
VerifyP256      41.0 ± 0%      24.0 ± 0%  -41.46%  (p=0.008 n=5+5)

Change-Id: Id526c30c9b04b2ad79a55d76cab0e30cc8d60402
Reviewed-on: https://go-review.googlesource.com/66230
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>