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

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>
Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33399
(cherry picked from commit b318480f27322c15c6b10c1bab4418e81d44b77e)