target/arm: NS BusFault on vector table fetch escalates to NS HardFault

In the M-profile architecture, when we do a vector table fetch and it
fails, we need to report a HardFault. Whether this is a Secure HF or
a NonSecure HF depends on several things. If AIRCR.BFHFNMINS is 0
then HF is always Secure, because there is no NonSecure HardFault.
Otherwise, the answer depends on whether the 'underlying exception'
(MemManage, BusFault, SecureFault) targets Secure or NonSecure. (In
the pseudocode, this is handled in the Vector() function: the final
exc.isSecure is calculated by looking at the exc.isSecure from the
exception returned from the memory access, not the isSecure input
argument.)

We weren't doing this correctly, because we were looking at
the target security domain of the exception we were trying to
load the vector table entry for. This produces errors of two kinds:
* a load from the NS vector table which hits the "NS access
to S memory" SecureFault should end up as a Secure HardFault,
but we were raising an NS HardFault
* a load from the S vector table which causes a BusFault
should raise an NS HardFault if BFHFNMINS == 1 (because
in that case all BusFaults are NonSecure), but we were raising
a Secure HardFault

Correct the logic.

We also fix a comment error where we claimed that we might
be escalating MemManage to HardFault, and forgot about SecureFault.
(Vector loads can never hit MPU access faults, because they're
always aligned and always use the default address map.)

Backports commit 51c9122e92b776a3f16af0b9282f1dc5012e2a19 from qemu

qemu/target/arm/m_helper.c

@@ -619,7 +619,11 @@ static bool arm_v7m_load_vector(ARMCPU *cpu, int exc, bool targets_secure,
         if (sattrs.ns) {
             attrs.secure = false;
         } else if (!targets_secure) {
-            /* NS access to S memory */
+            /*
+             * NS access to S memory: the underlying exception which we escalate
+             * to HardFault is SecureFault, which always targets Secure.
+             */
+            exc_secure = true;
             goto load_fail;
         }
     }