[GDBserver] Use pre-generated tdesc as test
    
    Now, these *-generate.c files are only used in GDBserver for unit test.
    If $development is false (in release), these *-generate.c files won't be
    used at all.
    
    gdb/gdbserver:
    
    2017-07-07  Yao Qi  <yao.qi@linaro.org>
    
    	* configure.srv: Set srv_i386_linux_regobj empty if $development
    	is false.
    	* linux-i386-ipa.c (initialize_low_tracepoint): Don't call
    	initialize_low_tdesc.
    	* linux-x86-low.c (initialize_low_arch): Wrap initialize_low_tdesc
    	with #if initialize_low_tdesc.
    	* linux-x86-tdesc-selftest.c: New file.
    	* linux-x86-tdesc.c: Move code to linux-x86-tdesc-selftest.c.