kbuild: Properly disable -Wunterminated-string-initialization for clang

kbuild: Properly disable -Wunterminated-string-initialization for clang

Clang and GCC have different behaviors around disabling warnings
included in -Wall and -Wextra and the order in which flags are
specified, which is exposed by clang's new support for
-Wunterminated-string-initialization.

  $ cat test.c
  const char foo[3] = "FOO";
  const char bar[3] __attribute__((__nonstring__)) = "BAR";

  $ clang -fsyntax-only -Wextra test.c
  test.c:1:21: warning: initializer-string for character array is too long, array size is 3 but initializer has size 4 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Wunterminated-string-initialization]
      1 | const char foo[3] = "FOO";
        |                     ^~~~~
  $ clang -fsyntax-only -Wextra -Wno-unterminated-string-initialization test.c
  $ clang -fsyntax-only -Wno-unterminated-string-initialization -Wextra test.c
  test.c:1:21: warning: initializer-string for character array is too long, array size is 3 but initializer has size 4 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Wunterminated-string-initialization]
      1 | const char foo[3] = "FOO";
        |                     ^~~~~

  $ gcc -fsyntax-only -Wextra test.c
  test.c:1:21: warning: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (4 chars into 3 available) [-Wunterminated-string-initialization]
      1 | const char foo[3] = "FOO";
        |                     ^~~~~
  $ gcc -fsyntax-only -Wextra -Wno-unterminated-string-initialization test.c
  $ gcc -fsyntax-only -Wno-unterminated-string-initialization -Wextra test.c

Move -Wextra up right below -Wall in Makefile.extrawarn to ensure these
flags are at the beginning of the warning options list. Move the couple
of warning options that have been added to the main Makefile since
commit e88ca24319e4 ("kbuild: consolidate warning flags in
scripts/Makefile.extrawarn") to scripts/Makefile.extrawarn after -Wall /
-Wextra to ensure they get properly disabled for all compilers.

Fixes: 9d7a0577c9db ("gcc-15: disable '-Wunterminated-string-initialization' entirely for now")
Link: https://github.com/llvm/llvm-project/issues/10359
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>