Also set $0 for binary files
remove `mrb_str_buf_cat`

It does the same as `mrb_str_cat`.
Merge pull request #2103 from titouanc/global-zero-bytecode

Also set $0 for binary files
Merge pull request #2104 from cremno/remove-mrb_str_buf_cat

remove `mrb_str_buf_cat`