Signed-off-by: Daniel Bovensiepen <daniel@bovensiepen.net>

Removed ununnecessary escape character '\' in libmruby.flags.mak

Rewrite String#prepend with Ruby

This issue was reported by https://hackerone.com/an0n-j

Fix #3357

Fix segv on str_buf_cat

Modify from nofree to embed string

Also more size checks added.

The issue was reported by https://hackerone.com/ston3

Check array max size

Fix #3354

Fix segv when primitive value

Fix #3352

Check overflow string length

Do nothing when empty string

Fix #3360

Fix #3361

This issue was reported by https://hackerone.com/mg36

Added example for cross-compiling on Renesas RX630

Reported by https://hackerone.com/haquaman.

Reported by Denis Kasak https://hackerone.com/dkasak