gdbsupport: Adapt construct_inferior_arguments

Allow construct_inferior_arguments to handle zero args
and have it return a std::string, similar to how
stringify_argv in gdbsupport/common-utils does.

Also, add a const qualifier for the second parameter,
since it is only read, not written to.

The intention is to replace existing uses of
stringify_argv by construct_inferior_arguments
in a subsequent step, since construct_inferior_arguments
properly handles special characters, while stringify_argv
doesn't.

gdbsupport/ChangeLog:

	* common-inferior.cc, common-inferior.h (construct_inferior_arguments):
	Adapt to handle zero args and return a std::string.
	Adapt call site.

Change-Id: I126c4390a1018c7527b0b8fd545252ab8a5a7adc