[Issue 18810] root/ctfloat depends upon backend

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18810

--- Comment #3 from kinke at gmx.net ---
Just to be clear, LDC itself has no need for ld_sprint or strtold_dm. We let
LLVM do the parsing and printing (in arbitrary precision).

DMD needs it for building with an LDC host compiler on Windows, as LDC uses a
double-precision `real` on Windows/MSVC targets.

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