kbuild: Use '-fms-anonymous-structs' if it is available

kbuild: Use '-fms-anonymous-structs' if it is available

Clang recently added '-fms-anonymous-structs' [1] to specifically enable
the Microsoft tagged anonymous structure / union extension, for which
the kernel added '-fms-extensions' in commit c4781dc3d1cf ("Kbuild:
enable -fms-extensions"). Switch to this more narrow option if it is
available, which would have helped avoid the issue addressed by
commit a6773e6932cb ("jfs: Rename _inline to avoid conflict with clang's
'-fms-extensions'"). GCC has talked about adding a similar flag [2] as
well but potentially naming it differently.

Move the selection of the flag to Kconfig to make it easier to use
cc-option (as CC_FLAGS_DIALECT may be used in arch Makefiles, which may
be too early for cc-option in Kbuild) and customize based on compiler
flag names.

Link: https://github.com/llvm/llvm-project/commit/c391efe6fb67329d8e2fd231692cc6b0ea902956
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123623
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Helge Deller <deller@gmx.de>  # parisc
Link: https://patch.msgid.link/20260223-fms-anonymous-structs-v1-2-8ee406d3c36c@kernel.org
Signed-off-by: Nicolas Schier <nsc@kernel.org>