gdb: Don't assume r_ldsomap when r_version > 1 on Linux

The r_ldsomap field is specific to Solaris (part of librtld_db), and
should never be accessed for Linux.  glibc is planning to add a field
to support multiple namespaces.  But there will be no r_ldsomap when
r_version is bumped to 2.  Add linux_[ilp32|lp64]_fetch_link_map_offsets
to set r_ldsomap_offset to -1 and use them for Linux targets.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28236