Avoid manual memory management of argv arrays in gdb/compile

Avoid manual memory management of argv arrays in gdb/compile

This changes gdb/compile to use gdb_argv directly, rather than
manually managing the arrays itself.  A few new helpers are added to
gdb_argv.

gdb/ChangeLog
2020-09-23  Tom Tromey  <tom@tromey.com>

	* utils.h (class gdb_argv): Add move operators.
	<append>: New methods.
	* compile/compile.c (build_argc_argv): Remove.
	(compile_args_argc): Remove.
	(compile_args_argv): Change type.
	(set_compile_args): Simplify.
	(append_args): Remove.
	(filter_args): Remove argcp parameter.
	(get_args): Return gdb_argv.  Simplify.
	(compile_to_object): Update.