sparc/build: Add SPARC target flags for compiling with clang

clang only supports building 64-bit kernel, so we use the
sparc64-linux-gnu target.

See also: https://lore.kernel.org/lkml/e26PTXUXEz8OYXmaeKn4Mpuejr4IOlFfFwdB5vpsluXlYiqDdlyQTYcDtdAny_o4gO4SfPeQCCN2qpyT6e0nog5EaP3xk2SeUPTrF54p1gM=@protonmail.com/T/#m068e010dcf8b99d3510a90d7532bcdb70e2e2c6b

Signed-off-by: Koakuma <[e-mail]>
Acked-by: Masahiro Yamada <[e-mail]>
Acked-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Andreas Larsson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Andreas Larsson <[e-mail]>

Documentation/kbuild/llvm.rst

···
    * - s390
      - Maintained
      - ``LLVM=1`` (LLVM >= 18.1.0), ``CC=clang`` (LLVM < 18.1.0)
+   * - sparc (sparc64 only)
+     - Maintained
+     - ``CC=clang LLVM_IAS=0`` (LLVM >= 20)
    * - um (User Mode)
      - Maintained
      - ``LLVM=1``

scripts/Makefile.clang

···
 CLANG_TARGET_FLAGS_powerpc	:= powerpc64le-linux-gnu
 CLANG_TARGET_FLAGS_riscv	:= riscv64-linux-gnu
 CLANG_TARGET_FLAGS_s390		:= s390x-linux-gnu
+CLANG_TARGET_FLAGS_sparc	:= sparc64-linux-gnu
 CLANG_TARGET_FLAGS_x86		:= x86_64-linux-gnu
 CLANG_TARGET_FLAGS_um		:= $(CLANG_TARGET_FLAGS_$(SUBARCH))
 CLANG_TARGET_FLAGS		:= $(CLANG_TARGET_FLAGS_$(SRCARCH))