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

This reverts commit 9ccf64e7 which is
upstream commit 18f14afe

.

Breaks the build:

arch/powerpc/kvm/book3s_hv_rmhandlers.S:2689: Error: operand out of range (0x0000000000008310 is not between 0xffffffffffff8001 and 0x0000000000008000)
make[3]: *** [scripts/Makefile.build:382: arch/powerpc/kvm/book3s_hv_rmhandlers.o] Error 1

Signed-off-by: Sasha Levin <sashal@kernel.org>