Change the default (minimum) client malloc alignment from 8 to 16

Change the default (minimum) client malloc alignment from 8 to 16
on ppc32-linux.  This is needed to make Altivec-using code work
correctly.  Noticed when running ./auxprogs/gsl16test with gcc-4.6
with args -mcpu=970 -g -O3 -ftree-vectorize on Memcheck, in
which case a few of the tests failed because malloc() returns
8 byte aligned memory when it should return 16-aligned memory.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11715