Add new tuning option: Constant-time branches

[stnolting]

New top generic to enable constant-time branches (data-independent timing):

CPU_CONSTT_BR_EN : boolean := false; -- implement constant-time branches

• When this option is enabled, all conditional branch instructions have identical execution times for taken and not taken branch conditions. Thus, all branches behave as if they were always taken (including a complete CPU pipeline flush). Enabling this feature makes execution times more predictable and makes timing side-channel attacks more difficult.
• When disabled, not-taken conditional branches are executed faster without clearing the CPU pipeline. Hence, for maximum performance, this feature should be disabled.