Handle multiple addresses in call_site_target

A large customer program has a function that is partitioned into hot
and cold parts.  A variable in a callee of this function is described
using DW_OP_GNU_entry_value, but gdb gets confused when trying to find
the caller.  I tracked this down to dwarf2_get_pc_bounds interpreting
the function's changes so that the returned low PC is the "wrong"
function.

Intead, when processing DW_TAG_call_site, the low PC of each range in
DW_AT_ranges should be preserved in the call_site_target.  This fixes
the variable lookup in the test case I have.

I didn't write a standalone test for this as it seemed excessively
complicated.