MIPS: make sure to fill 'jr' branch delay slot with 'nop'

Inline assembly in RoLO and the FiiO M3K bootloader used 'jr' to
jump to a newly loaded Rockbox binary, but incorrectly left the
branch delay slot open. That gives GCC an opening to place illegal
instrutions, etc, which might cause an unhandled exception.

Change-Id: Ia7a561fe530e94a41189d25f18a767c448177960

bootloader/fiiom3k-spl.c

@@ -199,6 +199,8 @@ void spl_main(void)
     /* Flush caches and jump to address */
     void* execaddr = (void*)opt->exec_addr;
     commit_discard_idcache();
-    __asm__ __volatile__ ("jr %0" :: "r"(execaddr));
+    __asm__ __volatile__ ("jr %0\n"
+                          "nop\n"
+                          :: "r"(execaddr));
     __builtin_unreachable();
 }

bootloader/fiiom3k.c

@@ -47,7 +47,9 @@ void exec(void* dst, const void* src, int bytes)
 {
     memcpy(dst, src, bytes);
     commit_discard_idcache();
-    __asm__ __volatile__ ("jr %0" :: "r"(dst));
+    __asm__ __volatile__ ("jr %0\n"
+                          "nop\n"
+                          :: "r"(dst));
     __builtin_unreachable();
 }
 

firmware/rolo.c

@@ -204,6 +204,7 @@ void rolo_restart(const unsigned char* source, unsigned char* dest,
     commit_discard_idcache();
     asm volatile(
         "jr     %0               \n"
+        "nop\n"
         : : "r"(dest)
     );
 #endif