ARM: include unprivileged BPF status in Spectre V2 reporting

The mitigations for Spectre-BHB are only applied when an exception
is taken, but when unprivileged BPF is enabled, userspace can
load BPF programs that can be used to exploit the problem.

When unprivileged BPF is enabled, report the vulnerable status via
the spectre_v2 sysfs file.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>