Share code updating gdb_target_obs

Share code updating gdb_target_obs

Nowadays, there are much duplications in configure.tgt to update
gdb_target_obs, some cpu specific object files are added to gdb_target_obs
to some different target triplets of the same cpu.  The same problem
exists for os specific object files too.  It is fragile to update them,
and build with all targets enabled doesn't find the problem.

This patch splits the gdb_target_obs update to three steps, cpu steps, os
steps, and the rest.

I tested this patch by build gdb for each different target triplets
respectively,

aarch64-elf aarch64-rtems aarch64-freebsd aarch64-linux alpha-elf
alpha-linux alpha-netbsd alpha-openbsd arm-elf arm-wince-pe arm-linux
arm-netbsd arm-symbianelf avr cris-elf frv-elf h8300-elf i386-elf
i386-darwin i386-dicos i386-freebsd i386-netbsdelf i386-openbsd
i386-nto i386-solaris i386-linux i386-gnu i386-cygwin i386-mingw32
i386-go32 ia64-linux-gnu ia64-vms lm32-elf m32c-elf m32r-elf m32r-linux
m68hc11-elf m68k-elf m68k-linux m68k-netbsd m68k-openbsd m88k-openbsd
mep-elf microblaze-xilinx-elf microblaze-linux-gnu mips-elf moxie-elf
ms1-elf nios2-elf nios2-linux-gnu hppa-elf hppa-linux hppa-netbsd
hppa-openbsd powerpc-eabi powerpc-freebsd powerpc-netbsd powerpc-openbsd
powerpc-linux powerpc-lynx178 rl78-elf rx-elf s390-linux-gnu score-elf
sh-elf sh-linux sh-openbsd sh64-elf sh64-linux sh64-openbsd sparc64-linux
sparc-linux sparc-freebsd sparc64-freebsd sparc-netbsd sparc64-netbsd
sparc-openbsd sparc64-openbsd spu-elf tic6x-elf tic6x-uclinux v850-elf
vax-netbsd vax-openbsd x86_64-linux-gnu x86_64-darwin x86_64-dicos
x86_64-elf x86_64-freebsd x86_64-mingw32 x86_64-netbsd x86_64-openbsd
x86_64-rtems xstormy16-elf xtensa-elf xtensa-linux

gdb:

2017-10-06  Yao Qi  <yao.qi@linaro.org>

	* configure.tgt (i386_tobjs): New variable.
	(amd64_tobjs): New variable.
	Set $cpu_obs and $os_obs.