Fix x86 build failures

Fix x86 build failures

Christophe Lyon told me that GDB build failed on i386-linux with
--enable-64-bit-bfd=yes, so I audit the gdb/configure.tgt again.  I
find that i386-darwin has the same issue too.  Additionally, GDB
for solaris target fails to build too.  This patch fixes all of them.

gdb:

2017-09-15  Yao Qi  <yao.qi@linaro.org>

	* configure.tgt (i[34567]86-*-darwin*): Append amd64.o to
	gdb_target_obs.
	(i[34567]86-*-solaris2.1[0-9]* | x86_64-*-solaris2.1[0-9]*):
	Likewise.
	(i[34567]86-*-linux*): Likewise.