PR13296: allow SDT operands using VARNAME

PR13296: allow SDT operands using VARNAME

We may encounter SDT operands of the form [OFF+]VARNAME[+OFF][(REG)]
which refers to the address of a object in the symtab. On x86_64, the
REG can be '%rip' to denote RIP-relative addressing.

To do this, we also pass the current dwflpp object to
sdt_uprobe_var_expanding_visitor, which then tries symbol resolution
using the symbol table if it meets upon such an SDT operand.