Fix Issue 21853 - std.format: formatting real.max in CTFE fails

[berni44]

I introduced this bug in #7862: The reason is, that log2 rounds up (due to floating points not precise enough) when the result is very close to an integer and thus the appended floor won't work anymore.

I fixed this by checking for max_exp - 1 explicitly and for all others by detecting, that the first (implied) bit of the mantissa is 0 and not 1 as it should be. In this case the result is corrected accordingly.

PS: I would really appreciate if we could have a cast(ulong[2]) for reals in CTFE, similar to the available casts for float and double. This would safe us from such trouble.

[dlang-bot]

Thanks for your pull request and interest in making D better, @berni44! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21853	normal	std.format: formatting real.max in CTFE fails

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8001"

[berni44]

Forgot to check for x87 reals in the unittest.