Remove duplicate builtins from std.math

[Geod24]

A few years ago, we started moving builtins from Phobos to druntime,
as it is where the compiler dependencies should live.
This was never completed, and some dependencies still live in Phobos,
while other definitions are missing from `core.math`
(e.g. unimplemented builtins are not defined).

While completing this will take time, a trivial first step is to remove
the functions that just forwards to the builtins.

Note: I counted the builtins, and there are 65 after the removal (and not 68).

Now for the sake of the reviewers:

• std.math.sin
• std.math.cos
• std.math.sqrt
• std.math.fabs
• std.math.yl2x and std.math.yl2xp1 are not even defined
• std.math.rndtol
• std.math.ldexp

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[thewilsonator]

Thats nice but I want to eventually move away from mangles for builtins/intrinsics, see e.g. #8002

[thewilsonator]

../../phobos/std/math.d(6695): Error: cannot convert `&real` to `ubyte*` at compile time

The offending line:

int signbit(X)(X x) @nogc @trusted pure nothrow
{
    alias F = floatTraits!(X);
    return ((cast(ubyte *)&x)[F.SIGNPOS_BYTE] & 0x80) != 0; // <<<<
}

seems to be the only error though.

[Geod24]

Yep it's tan. It's implemented with a CTFE path, but I doubt it's ever actually called (!) because the builtin takes priority. I'll take care of it in a later PR, re-added it for the time being.

Thats nice but I want to eventually move away from mangles for builtins/intrinsics, see e.g. #8002

Ah, good to know! I see both PRs as complementary, as I'm just trying to reduce the duplication and increase consistency.