BugLink: https://bugs.launchpad.net/bugs/1824864

All other archs have this value set higher, and the low value of 14
results in a log buffer so small it fills up before systemd-journald
can start and read all the boot time kernel log messages.

Increasing this will result in more memory reserved for the log buffer,
but will avoid missed kernel log messages.

This changes all 64 bit archs to use a shift of 18, which is what
amd64 has been using.
Signed-off-by: Dan Streetman <ddstreet@canonical.com>
Cc: Dimitri John Ledkov <xnox@ubuntu.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
[ kleber: fixed subject line. ]
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>