clang: Add $(CLANG_TARGET) to LDPPFLAGS

clang: Add $(CLANG_TARGET) to LDPPFLAGS

When we invoke $(CPP) to make u-boot.lds we have LDPPFLAGS available to
set other required flags here. As this file is for the target and not
the host, we must ensure that CPP knows what the target architecture is.
For this, pass in $(CLANG_TARGET).

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>