On ppc32/64 we don't have a reliable way to detect function entries

On ppc32/64 we don't have a reliable way to detect function entries
and exits at the moment.  So disable stack array bounds checking for
ppc32/64 platforms.  Also (unnecessarily) disables global array bounds
checking on those platforms.

Add a flag --enable-sg-checks=no|yes [yes] so that stack and global
checking can be optionally disabled on any platform.  This is useful
because stack and global checking is much more expensive than heap
checking, and so it may be desirable to disable it.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@8686