Phobos: Boost some transcendental math functions for float/double

[kinke]

Based on upstream PR dlang/phobos#6272.

[kinke]

lit-test tests/compilable/ctfe_math.d fails because the newly introduced, statically unrolled poly() version isn't CTFE-able (I guess due to passing a static immutable static array by ref).

[kinke]

That additional poly() version doesn't affect the optimized LDC timings for the transcendental math functions, i.e., the existing slice-version is apparently inlined and unrolled as it should.
So in theory I could just add that additional version for DMD only; the proper way to fix the failure is probably to wait for dlang/dmd#8071 and then add the new float/double versions as CTFE builtins too.

[kinke]

A (slightly updated) version of this is incorporated in branch ldc-merge-2.082, as the upstream pull was merged just in time.