LU-16058 build: proc_ops check fails with SUBARCH undefined

LU-16058 build: proc_ops check fails with SUBARCH undefined

During configure with config.cache enabled SUBARCH may not
be defined.

Move the definition to a location that must be traversed.

Lustre-change: https://review.whamcloud.com/48101
Lustre-commit: 8a7aa8d5902fcffe6c8b56d0c92ee36380a26e88

Test-Parameters: trivial
Fixes: a5084c2f2e ("LU-14937 build: re-use config cache in 'make rpms/debs'")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I0a7b4de3ecccd41b1c55e8b2df29039517e0c416
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/48423
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>