Update README with PowerPC specifics notes
RSS usage tuning for 64K pages

This patch lowers TCMalloc RSS usage when TCMALLOC_LARGE_PAGES64K is
defined by tuning page scavenge values.
Fix page_heap testcase for 64K pages

This patch fixes the page_heap testcase 32-bit build when
TCMALLOC_LARGE_PAGES64K is defined.