[testsuite] Add missing dg-require-effective-target alloca to gcc testsuite

[Mike Stump]

On Mar 23, 2017, at 8:46 AM, Tom de Vries <Tom_deVries@mentor.com> wrote:
> 
> I've run the gcc testsuite for target nvptx-none and ran into "test for excess errors" FAILs due to:
> ...
> sorry, unimplemented: target cannot support alloca.

We'd encourage ports to support alloca.  :-)

> OK for trunk for stage1?

Ok.  Ok for release branches and trunk as well, if you want.  I'd recommend trunk, if your port is meant to work and test out nicely in gcc 7.