build: fix building u-boot.lds with Clang as a cross-compiler

build: fix building u-boot.lds with Clang as a cross-compiler

Make sure to pass Clang flags to the KBUILD_CPPFLAGS as well, as this
variable is used for flags during compiling for target

Skipping this leads to Clang being invoked with args for target, but
without target indication, thus defaulting to host arch:

  LDS     u-boot.lds
clang: error: ... '-mabi=' for target 'x86_64-suse-linux'
clang: error: ... '-mno-thumb' for target 'x86_64-suse-linux'
clang: error: ... '-mno-unaligned-access' for target 'x86_64-suse-linux'
clang: error: ... '-ffixed-r9' for target 'x86_64-suse-linux'
clang: error: ... '-mno-movt' for target 'x86_64-suse-linux'
make: *** [Makefile:2345: u-boot.lds] Fehler 1

Signed-off-by: Dmitrii Sharshakov <d3dx12.xx@gmail.com>