tools/build: Reject unexpected values for LLVM=

tools/build: Reject unexpected values for LLVM=

Since commit 502678b88cb3 ("kbuild: Reject unexpected values for LLVM=")
the regular kbuild rejects unexpected values of LLVM=1 instead of
silently treating them as LLVM=1.

Align the tools build to kbuild.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260226-kbuild-llvm-followup-v1-2-201cc2a492d9@weissschuh.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>