x86/traps: const-correctness for IST handlers

x86/traps: const-correctness for IST handlers

NMI and MCE interrupt handlers have no right to modify their exception frame
or underlying vcpu registers.  Apply liberal quantities of 'const' to 'struct
cpu_user_regs *' throughout the codebase.

The Double Fault handler, while an IST handler, reloads some extra
architectural state back into its regs parameter.  As this is for printing
purposes and on a terminal error path, the const requirements for #DF are
relaxed.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>