Add i386.o to gdb_target_obs for x86_64-* targets

This patch fixes the build failure caused by 22916b07
(Convert the rest x86 target descriptions).

gdb:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* configure.tgt (gdb_target_obs): Add i386.o for x86_64-*
	targets.