gcc-15: make 'unterminated string initialization' just a warning

gcc-15 enabling -Wunterminated-string-initialization in -Wextra by
default was done with the best intentions, but the warning is still
quite broken.

What annoys me about the warning is that this is a very traditional AND
CORRECT way to initialize fixed byte arrays in C:

	unsigned char hex[16] = "0123456789abcdef";

and we use this all over the kernel.  And the warning is fine, but gcc
developers apparently never made a reasonable way to disable it.  As is
(sadly) tradition with these things.

Yes, there's "__attribute__((nonstring))", and we have a macro to make
that absolutely disgusting syntax more palatable (ie the kernel syntax
for that monstrosity is just "__nonstring").

But that attribute is misdesigned.  What you'd typically want to do is
tell the compiler that you are using a type that isn't a string but a
byte array, but that doesn't work at all:

	warning: ‘nonstring’ attribute does not apply to types [-Wattributes]

and because of this fundamental mis-design, you then have to mark each
instance of that pattern.

This is particularly noticeable in our ACPI code, because ACPI has this
notion of a 4-byte "type name" that gets used all over, and is exactly
this kind of byte array.

This is a sad oversight, because the warning is useful, but really would
be so much better if gcc had also given a sane way to indicate that we
really just want a byte array type at a type level, not the broken "each
and every array definition" level.

So now instead of creating a nice "ACPI name" type using something like

	typedef char acpi_name_t[4] __nonstring;

we have to do things like

	char name[ACPI_NAMESEG_SIZE] __nonstring;

in every place that uses this concept and then happens to have the
typical initializers.

This is annoying me mainly because I think the warning _is_ a good
warning, which is why I'm not just turning it off in disgust.  But it is
hampered by this bad implementation detail.

[ And obviously I'm doing this now because system upgrades for me are
  something that happen in the middle of the release cycle: don't do it
  before or during travel, or just before or during the busy merge
  window period. ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>