powerpc/boot: Set target when cross-compiling for clang

Clang needs to be told which target it is building for when cross
compiling.

Link: https://github.com/ClangBuiltLinux/linux/issues/259


Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Daniel Axtens <dja@axtens.net> # powerpc 64-bit BE
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>