Implement a more efficient allocation of small blocks which are never freed.
This generalises the "perm_malloc" function which was in ms_main.c
The new VG_(perm_malloc) is used in ms_main.c
and for execontext : when there are a lot of execontext, this
can save significant memory.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13238 a5019735-40e9-0310-863c-91ae7b9d1cf9