gdb: Add svr4-tls-tdep.o to list of objects of i*86 targets

GDB currently fails to build from source on i386 if compiled with
--enable-64-bit-bfd.  This is happening because svr4-tls-tdep.o is
missing from the gdb_target_obs variable, while amd64-linux-tdep.o is
obviously there.

Fix the problem by adding svr4-tls-tdep.o to the object list, but only
when --enable-64-bit-bfd is provided.

Signed-off-by: Sergio Durigan Junior <sergiodj@sergiodj.net>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33399