[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:

2017-09-05  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.