gdb/solib-svr4: check that solib is SVR4 in tls_maybe_fill_slot and tls_maybe_erase_slot

gdb/solib-svr4: check that solib is SVR4 in tls_maybe_fill_slot and tls_maybe_erase_slot

Functions tls_maybe_fill_slot and tls_maybe_erase_slot blindly assume
that the passe solibs come from solib-svr4.  This is not always the
case, because they are called even on the systems where the solib
implementation isn't solib-svr4.  Add some checks to return early in
that case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32990
Change-Id: I0a281e1f4826aa1914460c2213f0fae1bdc9af7c
Tested-By: Hannes Domani <ssbssa@yahoo.de>
Approved-By: Andrew Burgess <aburgess@redhat.com>