Fix Issue 20159 - Make std.math.scalbn work in CTFE

[n8sh]

No description provided.

[dlang-bot]

Thanks for your pull request, @n8sh!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20159	enhancement	Make std.math.scalbn work in CTFE

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + phobos#7153"

[WalterBright]

Getting:

std\math.d(4566): Error: static assert:  `nanL == infL` is false

now in Azure pipelines (Windows_VisualD win32-ldc)

[WalterBright]

@n8sh please fix.

[n8sh]

That's the same problem I was seeing with FreeBSD. It seemed like CTFE didn't have 80 bit reals even though they existed at runtime. I'll try enabling the slow path for all platforms.

[n8sh]

@WalterBright PR #7155