Fix block memory leak

-fsanitize=address report when CTRL+a x exiting:
=================================================================
==297977==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 23400 byte(s) in 117 object(s) allocated from:
    #0 0x761b706fd340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x633ad9c5e310 in block_translate src/emulate.c:649
    #2 0x633ad9c5e310 in block_find_or_translate src/emulate.c:865
    #3 0x633ad9c5e310 in rv_step src/emulate.c:1029
    #4 0x633ad9c5e310 in rv_run src/riscv.c:498
    #5 0x633ad9c5e310 in main src/main.c:279

Direct leak of 3136 byte(s) in 125 object(s) allocated from:
    #0 0x761b706fd9c7 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x633ad9c5fea4 in match_pattern src/emulate.c:767
    #2 0x633ad9c5fea4 in block_find_or_translate src/emulate.c:872
    #3 0x633ad9c5fea4 in rv_step src/emulate.c:1029
    #4 0x633ad9c5fea4 in rv_run src/riscv.c:498
    #5 0x633ad9c5fea4 in main src/main.c:279

Register a clean up callback, async_block_clear() to free all the
allocated memory fix this when emulator exits.

src/riscv.c

@@ -300,13 +300,34 @@ static void capture_keyboard_input()
 }
 #endif
 
+/*
+ *
+ * atexit() registers void (*)(void) callbacks, so no parameters can be passed.
+ * Memory must be freed at runtime. block_map_clear() requires a RISC-V instance
+ * and runs in interpreter mode. Instead of modifying its signature, access the
+ * global RISC-V instance in main.c with external linkage.
+ *
+ */
+extern riscv_t *rv;
+static void async_block_clear()
+{
+#if !RV32_HAS(JIT)
+    if (rv && rv->block_map.size)
+        block_map_clear(rv);
+#else  /* TODO: JIT mode */
+    return;
+#endif /* !RV32_HAS(JIT) */
+}
+
 riscv_t *rv_create(riscv_user_t rv_attr)
 {
     assert(rv_attr);
 
     riscv_t *rv = calloc(1, sizeof(riscv_t));
     assert(rv);
 
+    atexit(async_block_clear);
+
     /* copy over the attr */
     rv->data = rv_attr;