Simply use `snprintf` instead of custom `fmt_fp`,

Unless `MRB_DISABLE_STDIO` is set. `snprintf` is used anyway
if mruby is configured to use `stdio`. This change reduces 8KB
of program size on the Linux box.

src/fmt_fp.c

+#ifndef MRB_WITHOUT_FLOAT
+#ifdef MRB_DISABLE_STDIO
 /*
 
 Most code in this file originates from musl (src/stdio/vfprintf.c)
@@ -36,7 +38,6 @@ SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #include <mruby.h>
 #include <mruby/string.h>
 
-#ifndef MRB_WITHOUT_FLOAT
 struct fmt_args {
   mrb_state *mrb;
   mrb_value str;
@@ -371,4 +372,17 @@ mrb_float_to_str(mrb_state *mrb, mrb_value flo, const char *fmt)
   }
   return f.str;
 }
+#else   /* MRB_DISABLE_STDIO */
+#include <mruby.h>
+#include <stdio.h>
+
+mrb_value
+mrb_float_to_str(mrb_state *mrb, mrb_value flo, const char *fmt)
+{
+  char buf[24];
+
+  snprintf(buf, sizeof(buf), fmt, mrb_float(flo));
+  return mrb_str_new_cstr(mrb, buf);
+}
+#endif  /* MRB_DISABLE_STDIO */
 #endif