x86/build: Raise the minimum LLVM version to 15.0.0

In a similar vein as to this pending commit in the x86/asm tree:

a3e8fe814ad1 ("x86/build: Raise the minimum GCC version to 8.1")

... bump the minimum supported version of LLVM for building x86 kernels
to 15.0.0, as that is the first version that has support for
'-mstack-protector-guard-symbol', which is used unconditionally after:

80d47defddc0 ("x86/stackprotector/64: Convert to normal per-CPU variable"):

Older Clang versions will fail the build with:

clang-14: error: unknown argument: '-mstack-protector-guard-symbol=__ref_stack_chk_guard'

Fixes: 80d47defddc0 ("x86/stackprotector/64: Convert to normal per-CPU variable")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Brian Gerst <brgerst@gmail.com>
Link: https://lore.kernel.org/r/20250220-x86-bump-min-llvm-for-stackp-v1-1-ecb3c906e790@kernel.org