kbuild: clang: do not use CROSS_COMPILE for target triple

The target triple is overridden by the user-supplied CROSS_COMPILE,
but I do not see a good reason to support it. Users can use a new
architecture without adding CLANG_TARGET_FLAGS_*, but that would be
a rare case.

Use the hard-coded and deterministic target triple all the time.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>