[GDBserver] Replicate src dir in build dir

[GDBserver] Replicate src dir in build dir

Similar to f38307f5 (Replicate src dir in build dir), this patch change
configure and Makefile to generate object files in arch/ directory.

gdb/gdbserver:

2017-10-17  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (CONFIG_SRC_SUBDIR): New variable.
	(clean): Remove .o files in CONFIG_SRC_SUBDIR.
	(distclean): Remove DEPDIR in CONFIG_SRC_SUBDIR.
	(arch-i386.o, arch-amd64.o): Remove rules.
	(arch/%.o): New rule.
	Update POSTCOMPILE and COMPILE.pre.
	* configure.ac: Invoke AC_CONFIG_COMMANDS.
	* configure: Re-generated.
	* configure.srv: Replace arch-i386.o with arch/i386.o.
	Replace arch-amd64.o with arch/amd64.o.