gdb: remove gdbarch_info_init

While reviewing another patch, I realized that gdbarch_info_init could
easily be removed in favor of initializing gdbarch_info fields directly
in the struct declaration.  The only odd part is the union.  I don't
know if it's actually important for it to be zero-initialized, but I
presume it is.  I added a constructor to gdbarch_info to take care of
that.  A proper solution would be to use std::variant.  Or, these could
also be separate fields, the little extra space required wouldn't
matter.

gdb/ChangeLog:

	* gdbarch.sh (struct gdbarch_info): Initialize fields, add
	constructor.
	* gdbarch.h: Re-generate.
	* arch-utils.h (gdbarch_info_init): Remove, delete all usages.
	* arch-utils.c (gdbarch_info_init): Remove.

Change-Id: I7502e08fe0f278d84eef1667a072e8a97bda5ab5

gdb/corelow.c

@@ -165,7 +165,6 @@ core_target::core_target ()
   if (tdesc != nullptr)
     {
       struct gdbarch_info info;
-      gdbarch_info_init (&info);
       info.abfd = core_bfd;
       info.target_desc = tdesc;
       m_core_gdbarch = gdbarch_find_by_info (info);