powerpc/64s: Increase default stack size to 32KB

There are reports of kernels crashing due to stack overflow while
running OpenShift (Kubernetes). The primary contributor to the stack
usage seems to be openvswitch, which is used by OVN-Kubernetes (based on
OVN (Open Virtual Network)), but NFS also contributes in some stack
traces.

There may be some opportunities to reduce stack usage in the openvswitch
code, but doing so potentially require tradeoffs vs performance, and
also requires testing across architectures.

Looking at stack usage across the kernel (using -fstack-usage), shows
that ppc64le stack frames are on average 50-100% larger than the
equivalent function built for x86-64. Which is not surprising given the
minimum stack frame size is 32 bytes on ppc64le vs 16 bytes on x86-64.

So increase the default stack size to 32KB for the modern 64-bit Book3S
platforms, ie. pseries (virtualised) and powernv (bare metal). That
leaves the older systems like G5s, and the AmigaOne (pasemi) with a 16KB
stack which should be sufficient on those machines.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
Link: https://msgid.link/20231215124449.317597-1-mpe@ellerman.id.au