[ARM] Implement parse!real for systems with 64bit reals

Implement parse!real for systems where real==double.
This solution is probably not pretty, especially as it does not help on systems with 128bit reals or other sizes. However, it works fine and makes parse!real useable on ARM.

Note: This is part of a set of changes which are required to get test suite & unit tests passing on ARM GDC. All necessary changes for GDC are here for reference:
https://github.com/jpf91/GDC/commits/arm-old

Once these pull requests are merged upstream I'll merge them into GDC as well.