Add x86-tdep.o to i386/amd64 target build

Add x86-tdep.o to i386/amd64 target build

We get this error when doing a build with a single amd64 target (the
default when doing just ./configure on x86-64 GNU/Linux):

/home/simark/src/binutils-gdb/gdb/i386-tdep.c:4431: error: undefined reference to 'x86_in_indirect_branch_thunk(unsigned long, char const**, int, int)'
/home/simark/src/binutils-gdb/gdb/amd64-tdep.c:3045: error: undefined reference to 'x86_in_indirect_branch_thunk(unsigned long, char const**, int, int)'

The problem is that commit

  1d509aa625f8 ("infrun: step through indirect branch thunks")

missed adding x86-tdep.o to the list of object file included in an amd64
or i386 build.  The problem is not seen with --enable-targets=all
because that file is included in ALL_TARGET_OBS.

Built-tested using:

  * --host=x86_64-pc-linux-gnu --target=x86_64-pc-linux-gnu
  * --host=armv7-rpi2-linux-gnueabihf --target=x86_64-pc-linux-gnu

gdb/ChangeLog:

	* configure.tgt (x86_tobjs): New variable.
	(amd64_tobjs, i386_tobjs): Use it.