linux: Constify rfv variable in dl_vdso_vsym

Compilers may decide to put the rfv variable in .data rather than on
the stack.  It's slightly better to put it in .data.rel.ro.local
instead.  Regardles of that, making it const may enable further
optimisations.  Found when examining relative relocations (GOT ones
in particular) as part of enabling static-pie for PowerPC64.