package/xenomai: move arch restriction to Cobalt core, no restriction for Mercury

Xenomai has two mutually exclusive cores:
- Cobalt: dual-kernel approach: patched kernel + userland
- Mercury: only userland

In the Cobalt core, not all architectures are supported. This is the source
of the existing ARCH_SUPPORTS variable.

In the Mercury core, there is no imposed architecture restriction.

Rename the XENOMAI_ARCH_SUPPORTS flag to XENOMAI_COBALT_ARCH_SUPPORTS and
move its check from the Xenomai package to the Cobalt core.

Nevertheless, even for Mercury, there are some restrictions:
- pthread_atfork is used, which requires an MMU
- sync functions like __sync_sub_and_fetch and __sync_add_and_fetch are
  expected.

As the corresponding 'linux extension' selects Xenomai, we add the
MMU and sync dependencies there too. They may or may not already be covered
by XENOMAI_COBALT_ARCH_SUPPORTS flag.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>