Make M-bit dependent on X for RV64 (#550)

Fixes #549

This affects spec version 0.9.3

src/insns/acperm_32bit.adoc

@@ -61,7 +61,7 @@ The rules from <<acperm_rules>> must be followed when removing permissions.
 | 13  (RV32 only) | <<x_perm>>   | (<<c_perm>> and <<lm_perm>> and <<el_perm>> and (<<sl_perm>> == ∞)) or +
                                    (not(<<c_perm>> and not(<<lm_perm>>) and not(<<el_perm>>) and (<<sl_perm>>==0)))^1^
 | 14              | <<asr_perm>> | <<x_perm>>
-| 15^2^ (RV32 only) | <<m_bit>>  | <<x_perm>> and {cheri_default_ext_name} is implemented
+| 15^2^           | <<m_bit>>    | <<x_perm>> and {cheri_default_ext_name} is implemented
 |===
 
 ^1^ All the listed permissions in the set are either minimum or maximum. +