When running a simulation with, for example,
make posix jmavsim
px4 crashed almost 100% reproducable near start up.

This turned out to be a stack overflow. On gitter
it was suggested that the main reason for this could
be stack sizes, as currently used, assume a 32bit pointer
size and that doubling the stack size for everything
but NuttX would be the Right Thing.

This is the solution that I came up with (it makes
my core dumps disappear).