Uses/linux.mk: Restore i386 guard for linux-c7-*

Without this, when USE_LINUX_RPM=yes (LIB_DISTNAMES) is used with linux:c7 and
the port sets ONLY_FOR_ARCHS=amd64, the i686 RPM is still being added to
DISTFILES_amd64, causing fetching a file not in distinfo.

This fixes devel/linux-ltp build.

Reviewed by: arrowd
Fixes: 5aa75e1ca0fc Linuxulator ports: Introduce Rocky Linux 9
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56796