gdb: remove inferior::{argc,argv}

gdb: remove inferior::{argc,argv}

There are currently two states that the inferior args can be stored.
The main one is the `args` field, where they are stored as a single
string.  The other one is the `argc`/`argv` fields.

This last one is only used for arguments passed in GDB's
command line.  And the only outcome is that when get_inferior_args is
called, `argc`/`argv` are serialized into `args`.  So really,
`argc`/`argv` is just a staging area before moving the arguments in
`args`.

Simplify this by only keeping the `args` field.  Change
set_inferior_args_vector to immediately serialize the arguments into
`args`, work that would be done in get_inferior_args later anyway.

The only time where this work would be "wasted" is when the user passes
some arguments on the command line, but does not end up running the
program.  But that just seems unlikely.  And it's not that much work.

Change-Id: Ica0b9859397c095f6530350c8fb3c36905f2044a