telemetry: fix build for armv7

telemetry: fix build for armv7

telemetry can not depend on EAL anymore but it still wants to get arch
headers.
We directly point at the right source directories by using the same logic
than EAL. However the special case of armv7 has been missed.

Fix this by defaulting ARCH_DIR to RTE_ARCH.

Caught on OBS:
[  162s]   SYMLINK-FILE include/rte_telemetry.h
[  162s]   CC telemetry.o
[  162s]   CC telemetry_data.o
[  162s]   CC telemetry_legacy.o
[  162s] .../lib/librte_telemetry/telemetry.c:15:10: fatal error:
 rte_spinlock.h: No such file or directory
[  162s]  #include <rte_spinlock.h>
[  162s]           ^~~~~~~~~~~~~~~~
[  162s] compilation terminated.

Fixes: 6dd571fd07c3 ("telemetry: introduce new functionality")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>