Add support for controlling the pointer masking mode on harts which
support the Smnpm extension. This extension can only exist on harts
where XLEN >= 64 bits. This implementation selects the mode with the
smallest PMLEN that satisfies the caller's requested lower bound.
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>