semihosting: Use struct gdb_stat in common_semi_flen_cb

Load the entire 64-bit size value.  While we're at it,
use offsetof instead of an integer constant.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

semihosting/arm-compat-semi.c

@@ -325,14 +325,12 @@ static void
 common_semi_flen_cb(CPUState *cs, target_ulong ret, target_ulong err)
 {
     if (!err) {
-        /*
-         * The size is always stored in big-endian order, extract
-         * the value. We assume the size always fit in 32 bits.
-         */
-        uint32_t size;
-        cpu_memory_rw_debug(cs, common_semi_flen_buf(cs) + 32,
-                            (uint8_t *)&size, 4, 0);
-        ret = be32_to_cpu(size);
+        /* The size is always stored in big-endian order, extract the value. */
+        uint64_t size;
+        cpu_memory_rw_debug(cs, common_semi_flen_buf(cs) +
+                            offsetof(struct gdb_stat, gdb_st_size),
+                            &size, 8, 0);
+        ret = be64_to_cpu(size);
     }
     common_semi_cb(cs, ret, err);
 }