Fix a buffer overrun on Linux/PPC64.

The buffer in `realToMangleBuffer()` is too small for longdouble values. It produces strings like `0X1.599999999999999999999999998P+1` or `0X1.999999999999999999999999998P-4` which have a length of 34. This results in an ICE e.g. in the unit test of std.range.