Improve String#rindex performance on OSX

On OSX, String#rindex is slow due to the lack of `memrchr`.
The fallback implementation finds a match by instead doing
a `memcmp` on every single character in the search string
looking for a substring match.

For OSX hosts, this changeset introduces a simple `memrchr`
implementation, `rb_memrchr`, that can be used instead. An
example benchmark below demonstrates an 8000 char long
search string with a 10 char substring near the end.

```
ruby-master | substring near the end | osx

UTF-8
       user     system      total        real
index  0.000111   0.000000   0.000111 (  0.000110)
rindex  0.000446   0.000005   0.000451 (  0.000454)
```

```
ruby-patched | substring near the end | osx

UTF-8
       user     system      total        real
index  0.000112   0.000000   0.000112 (  0.000111)
rindex  0.000057   0.000001   0.000058 (  0.000057)
```

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