lib/utils/pyexec: Condition-out GC calls from pyexec.

A port which uses lib/utils/pyexec.c but which does not enable garbage
collection should not need to implement the gc_collect function.

This patch also moves the gc_collect call to after printing the qstr
info.  Since qstrs cannot be collected it should not make any difference
to the printed statistics.

bare-arm/main.c

@@ -34,9 +34,6 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-void gc_collect(void) {
-}
-
 mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
     return NULL;
 }

lib/utils/pyexec.c

@@ -110,16 +110,18 @@ STATIC int parse_compile_execute(void *source, mp_parse_input_kind_t input_kind,
     if ((exec_flags & EXEC_FLAG_ALLOW_DEBUGGING) && repl_display_debugging_info) {
         mp_uint_t ticks = mp_hal_ticks_ms() - start; // TODO implement a function that does this properly
         printf("took " UINT_FMT " ms\n", ticks);
-        gc_collect();
         // qstr info
         {
             mp_uint_t n_pool, n_qstr, n_str_data_bytes, n_total_bytes;
             qstr_pool_info(&n_pool, &n_qstr, &n_str_data_bytes, &n_total_bytes);
             printf("qstr:\n  n_pool=" UINT_FMT "\n  n_qstr=" UINT_FMT "\n  n_str_data_bytes=" UINT_FMT "\n  n_total_bytes=" UINT_FMT "\n", n_pool, n_qstr, n_str_data_bytes, n_total_bytes);
         }
 
-        // GC info
+        #if MICROPY_ENABLE_GC
+        // run collection and print GC info
+        gc_collect();
         gc_dump_info();
+        #endif
     }
 
     if (exec_flags & EXEC_FLAG_PRINT_EOF) {