Use visitor pattern for "maint print c-tdesc"

Target description can be modeled as a tree, the target description
is the root node, features are children nodes, registers and types are
grand-children nodes.  So command "maint print c-tdesc" in effect
traverse/visit each node, and print them in c.  This can be
implemented by visitor pattern, this is the first reason.  Secondly,
I want to this command prints c files in a different way for some
specific xml files, but still print c files the same way for the rest
of xml files.  Third, I even want to print xml files from target
descriptions, so that GDBserver can use it to reply GDB's query
qXfer:features:read:target.xml.

gdb:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* target-descriptions.c (tdesc_element_visitor): New class.
	(tdesc_element): New class.
	(tdesc_reg): Inherit from tdesc_element.
	(tdesc_reg::accept): New function.
	(tdesc_type): Inherit from tdesc_element.
	(tdesc_type::accept): New function.
	(tdesc_feature): Inherit from tdesc_element.
	(tdesc_feature::accept): New function.
	(target_desc): Inherit from tdesc_element.
	(target_desc::target_desc): New.
	(target_desc::~target_desc): New.
	(target_desc::accept): New.
	(allocate_target_description): Use new.
	(free_target_description): Use delete.
	(print_c_tdesc): New class.
	(maint_print_c_tdesc_cmd): Adjust.

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	* features/i386/amd64-avx-avx512-linux.c: Re-generated.
	* features/i386/amd64-avx.c: Re-generated.
	* features/i386/amd64-avx-linux.c: Re-generated.
	* features/i386/amd64-avx-mpx-avx512-pku.c: Re-generated.
	* features/i386/amd64-avx-mpx-avx512-pku-linux.c: Re-generated.
	* features/i386/amd64-avx-mpx.c: Re-generated.
	* features/i386/amd64-avx-mpx-linux.c: Re-generated.
	* features/i386/amd64.c: Re-generated.
	* features/i386/amd64-linux.c: Re-generated.
	* features/i386/amd64-mpx.c: Re-generated.
	* features/i386/amd64-mpx-linux.c: Re-generated.
	* features/i386/i386-avx-avx512.c: Re-generated.
	* features/i386/i386-avx-avx512-linux.c: Re-generated.
	* features/i386/i386-avx.c: Re-generated.
	* features/i386/i386-avx-linux.c: Re-generated.
	* features/i386/i386-avx-mpx-avx512-pku.c: Re-generated.
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	* features/i386/i386-avx-mpx.c: Re-generated.
	* features/i386/i386-avx-mpx-linux.c: Re-generated.
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	* features/i386/i386-linux.c: Re-generated.
	* features/i386/i386-mmx.c: Re-generated.
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	* features/i386/i386-mpx.c: Re-generated.
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	* features/rs6000/powerpc-isa205-altivec32l.c: Re-generated.
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	* features/rs6000/powerpc-isa205-vsx32l.c: Re-generated.
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	* features/tic6x-c64xp.c: Re-generated.
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1 files changed, 4 insertions, 4 deletions

diff --git a/gdb/features/i386/i386-mmx.c b/gdb/features/i386/i386-mmx.c
index 74f67ed..02aee89 100644
--- a/gdb/features/i386/i386-mmx.c
+++ b/gdb/features/i386/i386-mmx.c
@@ -10,13 +10,13 @@ static void
 initialize_tdesc_i386_mmx (void)
 {
   struct target_desc *result = allocate_target_description ();
-  struct tdesc_feature *feature;
-  struct tdesc_type *field_type;
-  struct tdesc_type *type;
-
   set_tdesc_architecture (result, bfd_scan_arch ("i386"));
 
+  struct tdesc_feature *feature;
+
   feature = tdesc_create_feature (result, "org.gnu.gdb.i386.core");
+  struct tdesc_type *field_type;
+  struct tdesc_type *type;
   type = tdesc_create_flags (feature, "i386_eflags", 4);
   tdesc_add_flag (type, 0, "CF");
   tdesc_add_flag (type, 1, "");