[ARM] Implement remaining std.math stuff

Implement ARM support in std.math. This pull request only contains the compiler-independent parts. Some inline-asm is necessary to make std.math work 100% but we won't upstream this code. The gdc implementation is here, for reference:
https://github.com/jpf91/GDC/commit/017d7aa5d0745188f3d74ae068ce6c046adb4494

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.