exec, kvm, target-ppc: Move getrampagesize() to common code

getrampagesize() returns the largest supported page size and mainly
used to know if huge pages are enabled.

However is implemented in target-ppc/kvm.c and not available
in TCG or other architectures.

This renames and moves gethugepagesize() to mmap-alloc.c where
fd-based analog of it is already implemented. This renames and moves
getrampagesize() to exec.c as it seems to be the common place for
helpers like this.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>