devel/freebsd-sysroot: disable armv6 and armv7 flavors on big-endian architectures

Extracting little-endian UFS fails on big-endian systems, since UFS is not bi-endian:

> SHA256 Checksum OK for rust/bootstrap/armv6/12.2-RELEASE/FreeBSD-12.2-RELEASE-arm-armv6-RPI-B.img.xz.

6188032+0 records in
6188032+0 records out
3168272384 bytes transferred in 143.411914 secs (22092114 bytes/sec)
fstab: /etc/fstab:0: No such file or directory

DUMP: Can't open /etc/fstab for dump table information: No such file or directory
DUMP: Date of this level 0 dump: Mon Oct 18 18:22:26 2021
DUMP: Date of last level 0 dump: the epoch
DUMP: Dumping /wrkdirs/usr/ports/devel/freebsd-sysroot/work-armv6/base.img to standard output

dump: Cannot find file system superblock: No such file or directory
Tape is not a dump tape

• Error code 1