ARM: hide mach-*/ include for ARM_SINGLE_ARMV7M

The machine specific header files are exported for traditional
platforms, but not for the ones that use ARCH_MULTIPLATFORM, as
they could conflict with one another.

In case of ARM_SINGLE_ARMV7M, we end up also exporting them,
but that appears to be a mistake, and we should treat it the
same way as ARCH_MULTIPLATFORM here.

'make W=1' warns about this because it passes -Wmissing-includes
to gcc and the directories are not actually present.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>