Use construct_inferior_arguments which handles special chars

Use construct_inferior_arguments which handles special chars

Use the construct_inferior_arguments function instead of
stringify_argv to construct a string from the program
arguments in those places where that one is then passed
to fork_inferior (linux-low, lyn-low), since
construct_inferior_arguments properly takes care of
special characters, while stringify_argv does not.
Using construct_inferior_arguments seems "natural", since its
documentation also mentions that it "does the
same shell processing as fork_inferior".

Since construct_inferior_args has been extended to do
proper quoting for Windows shells in commit
5d60742e2dd3c9b475dce54b56043a358751bbb8
("Fix quoting of special characters for the MinGW build.",
2012-06-12), use it for the Windows case as well.
(I could not test that case myself, though.)

Adapt handling of empty args in function 'handle_v_run'
in gdbserver/server.cc to just insert an empty string
for an empty arg, since that one is now properly handled
in 'construct_inferior_arguments' already (and inserting
a "''" string in 'handle_v_run' would otherwise
cause that one to be treated as a string literally
containing two quote characters, which
'construct_inferior_args' would preserve by adding
extra escaping).

This makes gdbserver properly handle program args containing special
characters (like spaces), e.g. (example from PR25893)

  $ gdbserver localhost:50505 myprogram "hello world"

now properly handles "hello world" as a single arg, not two separate
ones ("hello", "world").

gdbserver/ChangeLog:

	PR gdbserver/25893
	* linux-low.cc (linux_process_target::create_inferior),
	lynx-low.cc (lynx_process_target::create_inferior),
	win32-low.cc (win32_process_target::create_inferior): Use
	construct_inferior_arguments instead of stringify_argv
	to get string representation which properly escapes
	special characters.
	* server.cc (handle_v_run): Just pass empty program arg
	as such, since any further processing is now handled via
	construct_inferior_arguments.

Change-Id: Ibf963fcd51415c948840fb463289516b3479b0c3