kbuild: Reject unexpected values for LLVM=

The LLVM argument is documented to accept one of three forms:
* a literal '1' to use the default 'clang',
* a toolchain prefix path, ending in a trailing '/',
* a version suffix.

All other values are silently treated as '1'. If for example
the user accidentally forgets the trailing '/' of a toolchain prefix,
kbuild will unexpectedly and silently fall back to the system toolchain.

Instead report an error if the user specified an invalid value for LLVM.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260121-kbuild-llvm-arg-v2-1-5e4d8dca4ad8@weissschuh.net
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Makefile

@@ -447,6 +447,8 @@ ifneq ($(filter %/,$(LLVM)),)
 LLVM_PREFIX := $(LLVM)
 else ifneq ($(filter -%,$(LLVM)),)
 LLVM_SUFFIX := $(LLVM)
+else ifneq ($(LLVM),1)
+$(error Invalid value for LLVM, see Documentation/kbuild/llvm.rst)
 endif
 
 HOSTCC	= $(LLVM_PREFIX)clang$(LLVM_SUFFIX)