Remove function VG_(sysnum_string_extra) as it was just a wrapper
around VG_(sysnum_string). Also remove associated macro
VG_SYSNUM_STRING_EXTRA.
The VG_SYSNUM_STRING macro returned a pointer to a variable which 
is out of scope. Using that value may cause undefined behaviour.
Change VG_(sysnum_string) to return pointer to static buffer instead.
Fix call sites.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14264 a5019735-40e9-0310-863c-91ae7b9d1cf9