Double check x87 precision correctness

x86 manual had a couple statements that caught my eye.

> The Precision control (PC) field comprises bits [9:8] of the x87 control word (“x87 Control Word Register (FCW)” on page 292). This field specifies the precision of floating-point calculations for the FADDx, FSUBx, FMULx, FDIVx, and FSQRT instructions, as shown in Table 6-11.

I think we modify precision on more than those instructions listed. Might need to check that.

Also:
> x87 computations are carried out in double-extended-precision format, so that the transcendental functions provide results accurate to within one unit in the last place (ulp) for each of the floating- point data types.

Might want to double check that we have the transcendental accuracy correct.