Remove current_exception_sp0.

[qwandor]

Switching spsel manually appears to be unnecessary. Exception vectors are always called with SP_ELx, and the spsel is restored from SPSR on eret.

[m4tx]

For posterity:

Exception handler pseudocode contains the following:

noreturn func AArch64_TakeException(target_el : bits(2), exception_in : ExceptionRecord,
                                    preferred_exception_return : bits(64), vect_offset_in : integer)
begin
    // ...
    PSTATE.SP = '1';

    // ...
    BranchTo{64}(VBAR_ELx()[63:11]::vect_offset[10:0], BranchType_EXCEPTION, branch_conditional);
    // ...
end;

eret instruction pseudocode contains the following:

AArch64.ExceptionReturn(bits(64) new_pc_in, bits(64) spsr,
                        boolean is_texit, boolean illegal_psr_state)
    // ...
    SetPSTATEFromPSR(spsr, illegal_psr_state, is_texit);
    // ...

SetPSTATEFromPSR(bits(N) spsr_in, boolean illegal_psr_state, boolean is_texit)
    // ...
    if illegal_psr_state then
        // ...
    else
        // ...
        PSTATE.SP  = spsr<0>;
    // ...

Hence, exception handler will always set spsel to 1 on enter and spsr<0> on exit (assuming eret is executed).