kbuild: create symbolic link only for ARM, AVR32, SPARC, PowerPC, x86

The symbolic link to SoC/CPU specific header directory is created
during the build, while it is only necessary for ARM, AVR32, SPARC,
x86, and some CPUs of PowerPC.  For the other architectures, it just
results in a broken symbolic link.

Introduce CONFIG_CREATE_ARCH_SYMLINK to not create unneeded symbolic
links.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>