arm: config: Introduce CONFIG_SYS_ARM_ARCH

Introduce new helper Kconfig option, which is automatically set to
the version of ARM architecture for which the U-Boot is built. This
is useful when selecting tuning options in the libgcc imported from
Linux kernel.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>