nto_process_target::create_inferior: Pass args as char **

According to [1], the fifth parameter
to the 'spawnp' function is 'char * const argv[]',
so just pass the args contained in the vector as
an array right away, rather than converting that
to a C string first and passing that one.

With commit 2090129c
("Share fork_inferior et al with gdbserver",
2016-12-22) the type had changed from 'char **'
to 'char *', but I can't see an apparent reason for
that, and 'nto_procfs_target::create_inferior'
(in gdb/nto-procfs.c) also passes a 'char **' to
'spawnp' instead.

I do not know much about that target and cannot actually
test this, however.
The main motivation to look at this was identifying
and replacing the remaining uses of the 'stringify_argv'
function which does not properly do escaping.

[1] http://www.qnx.com/developers/docs/7.0.0/#com.qnx.doc.neutrino.lib_ref/topic/s/spawnp.html

gdbserver/ChangeLog:

	* nto-low.cc (nto_process_target::create_inferior): Pass
	argv to spawnp function as char **.

Change-Id: Ic46fe745c2aa1118114240d149d4156032f84344