kbuild: Bump minimum version of LLVM for building the kernel to 15.0.0

s390 and x86 have required LLVM 15 since

  30d17fac ("scripts/min-tool-version.sh: raise minimum clang version to 15.0.0 for s390")
  7861640a ("x86/build: Raise the minimum LLVM version to 15.0.0")

respectively but most other architectures allow LLVM 13.0.1 or newer. In
accordance with the recent minimum supported version of GCC bump that
happened in

  118c40b7 ("kbuild: require gcc-8 and binutils-2.30")

do the same for LLVM to 15.0.0.

Of the supported releases of Arch Linux, Debian, Fedora, and OpenSUSE
surveyed in evaluating this bump, this only leaves behind Debian
Bookworm (14.0.6) and Ubuntu Jammy (14.0.0). Debian Trixie has 19.1.7
and Ubuntu Noble has 18.1.3 (so there are viable upgrade paths) or users
can use apt.llvm.org, which provides even newer packages for those
distributions.

Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250821-bump-min-llvm-ver-15-v2-1-635f3294e5f0@kernel.org


Signed-off-by: Nathan Chancellor <nathan@kernel.org>