[ARM] Fix phobos unit tests

Changes to make the unit tests pass on ARM. Mostly reducing required precision for math tests,
adding replacements for tests which required 80 bit reals and so on.

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.