posix: align mailboxes

Using unaligned mailboxes with UBSAN causes fuzzing failures with
aligned data structures

Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>

src/platform/posix/posix.c

@@ -9,10 +9,10 @@
 #include <sof/schedule/ll_schedule.h>
 #include <sof/lib/agent.h>
 
-uint8_t posix_hostbox[MAILBOX_HOSTBOX_SIZE];
-uint8_t posix_dspbox[MAILBOX_DSPBOX_SIZE];
-uint8_t posix_stream[MAILBOX_STREAM_SIZE];
-uint8_t posix_trace[MAILBOX_TRACE_SIZE];
+uint8_t posix_hostbox[MAILBOX_HOSTBOX_SIZE] __aligned(8);
+uint8_t posix_dspbox[MAILBOX_DSPBOX_SIZE] __aligned(8);
+uint8_t posix_stream[MAILBOX_STREAM_SIZE] __aligned(8);
+uint8_t posix_trace[MAILBOX_TRACE_SIZE] __aligned(8);
 
 /* This seems like a vestige. Existing Zephyr platforms are emitting
  * these markers in their linker scripts, and wrapper.c code iterates