/etc/fstab

# allgemein
tmpfs /tmp tmpfs rw,nosuid,noatime,nodev,size=4G,mode=1777 0 0

# mount -o remount,size=N /var/tmp/portage

/etc/fstab

# portage:
tmpfs /var/tmp/portage tmpfs rw,nosuid,noatime,nodev,size=4G,mode=775,uid=portage,gid=portage,x-mount.mkdir=775 0 0

/etc/portage/env/notmpfs.conf

PORTAGE_TMPDIR="/var/tmp/notmpfs"

# mkdir /var/tmp/notmpfs
# chown portage:portage /var/tmp/notmpfs
# chmod 775 /var/tmp/notmpfs

/etc/portage/package.env

app-office/libreoffice notmpfs.conf
mail-client/thunderbird notmpfs.conf
sys-devel/gcc notmpfs.conf