True pure std.math

There's going to be several more commits following what has already been included in this pull, so don't merge just yet.  Going to have these reviewed at a steady rate, ensuring that tests are passing just fine at each step.

This sets out to make the math functions in std.math pure as in it compiles and is implemented correctly for targets that don't define D_InlineAsm (eg: gdc, maybe non-x86 ldc)...

The following implementations are for 80-bit reals, though some produce not quite identical results as x86/x86_64 assembly implementations.  But are at least correct up to 64-bit precision depending on the implementation's peak and mean relative errors in over various domains.