Compile everything with -fno-builtin, so as to disable LLVM's

Compile everything with -fno-builtin, so as to disable LLVM's
idiom-recognition optimisation.  This identifies memset-style
loops and turns them into calls to memset, which in this case
leads to infinite recursion because it does this transformations
in VG_(memset), which is called from memset(), which we also define.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12065