test: mb_ereg regression

current git master fails to run the tests mb_ereg_replace_variation1.php
and mb_ereg_variation3.php on (at least) aarch64 Ubuntu Xenial (see [1])
and aarch64 OpenSuse LEAP 42.2.
This regression has been popped up within the last days and can be reliably
reproduced.

The build has been done using this commands:

```
rm -f ~/.hhvm.hhbc
git clean -dfx
rm -rf third-party
git submodule update --init --recursive
cmake -DENABLE_LD_GOLD=Off .
make -j8
./hphp/hhvm/hhvm hphp/test/run all
```

The test diffs show the following:

```
FAILED: hphp/test/zend/good/ext/mbstring/tests/mb_ereg_replace_variation1.php
013+ string(10) "string_val"
014- Warning\: [^\r\n]+
015- bool\(false\)
016-
019+ string(10) "string_val"
022- Warning\: [^\r\n]+
023- bool\(false\)
024-
```

```
FAILED: hphp/test/zend/good/ext/mbstring/tests/mb_ereg_variation3.php
053+ int(10)
053- int\(9\)
056+ string(16) "5pel5pys6KqeCQ=="
056- string\(12\) "5pel5pys6Kqe"
```