Added option to disable libunwind linking

This patch adds a configure option to enable or disable libunwind linking.
The patch also disables libunwind on ppc by default.
New compiler flags to set the size and alignment of tcmalloc pages

Added two new compiler flags, --with-tcmalloc-pagesize and
--with-tcmalloc-alignment, in order to set the tcmalloc internal page
size and alignment without the need of a compiler directive and to
make the choice of the page size independent of the 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.