remove theoretical uninitialised variable use in infer_cpu_gic_id

The C semantics used by our binary correctness mechanism understand
uninitialised variables as non-deterministic assignment. The translation
to a simplified form suitable for further analysis occurs on a
per-function basis which does not support non-determinism. This means
uninitialised local variables must not be used for decision making in
any function.

In infer_cpu_gic_id, 'target' is initialised in the loop, which will not
be executed if nirqs <= 0, after which the uninitialised 'target' is
examined. We address this by initialising 'target' to 0.

The overall C code is still safe, as infer_cpu_gic_id is only called
once in dist_init, where 0 < nirqs.

src/arch/arm/machine/gic_pl390.c

@@ -53,7 +53,7 @@ BOOT_CODE static uint8_t
 infer_cpu_gic_id(int nirqs)
 {
     word_t i;
-    uint32_t target;
+    uint32_t target = 0;
     for (i = 0; i < nirqs; i += 4) {
         target = gic_dist->targets[i >> 2];
         target |= target >> 16;