ctf: write the CTF files for standalone modules to a subdir of the module dir

ctf: write the CTF files for standalone modules to a subdir of the module dir

Writing all ctf to the .ctf subdirectory of the kernel directory is problematic
when building standalone modules, when the kernel directory may be unwritable.
So write it to the .ctf subdirectory of the module directory in this case
as well.  (The old .ctf relative path was hardwired into dwarf2ctf, so this
too is changed to accept the path to write the CTF files to as the first
parameter, in both non-standalone and standalone mode.)

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>