New configure flags to set the alignment and page size of tcmalloc

Added two new configure flags, --with-tcmalloc-pagesize and
--with-tcmalloc-alignment, in order to set the tcmalloc internal page
size and tcmalloc allocation alignment without the need of a compiler
directive and to make the choice of the page size independent of the
allocation alignment.
Make PPC64 use 64K of internal page size for tcmalloc by default

This patch set the default tcmalloc internal page size to 64K when
built on PPC.