Fix G2/G3 rounding error

[edwilliams16]

Description

This fixes the problems from rounding error discussed in #14745
and an error introduced in the refactoring 56595a4

Related Issues

fixes #14745

[thinkyhead]

Good fix, thanks!

[Evg33]

DUE_USB FAILED

Marlin\src\gcode\motion\G2_G3.cpp: In static member function 'static void GcodeSuite::G2_G3(bool)':
Marlin\src\gcode\motion\G2_G3.cpp:293:46: error: expected ',' or ';' before '/' token
           const xy_pos_t s = { -d2.y, d2.x } / len;      // Unit vector along perpendicular bisector
                                              ^
*** [.pio\build\DUE_USB\src\src\gcode\motion\G2_G3.cpp.o] Error 1

[edwilliams16]

{ -d2.y, d2.x } / len didn't compile for me either
{-d2.y, d2.x} * RECIPROCAL(len) looked slower (1 divide, 2 multiplies instead of two divides)

so I used {-d2.y/len, d2.x/len}
The alternative of overloading / for the xy_pos_t type was more than I wanted to tackle for this instance, as it would likely result in the same compiled code.

[thinkyhead]

Casting was needed. xy_pos_t({ -d2.y, d2.x }) / len. But we just moved the divide to the next line.