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 <koachan@protonmail.com>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/20241029-sparc-cflags-v3-2-b28745a6bd71@protonmail.com


Signed-off-by: Andreas Larsson <andreas@gaisler.com>