libmudflap emits a global initializer that registers memory ranges for
global data symbols.  However, even if IPA decides not to emit a symbol
because it's unused, we'd still emit registration sequences for them in
some cases, which, in the PR testcase, would result in TOC references to
the undefined symbols.

This patch fixes the problem, avoiding registration for symbols that are
not present in the varpool.

Regstrapped on x86_64-linux-gnu and i686-linux-gnu; I've also verified
that it removes the TOC references on a ppc64-linux-gnu cross.

Ok to install?