Add support for DSECNDS, the double-precision variant of SECNDS.
The implementation mirrors SECNDS, reusing the shared `SecndsImpl<T>`
runtime template.
Includes:
- Registration in intrinsics table
- Lowering handler and runtime call wiring
- Hook into shared SecndsImpl in extensions.cpp
- Documentation in Intrinsics.md
- Regression test dsecnds.f90
CC @eugeneepshteyn @klausler
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Co-authored-by: Eugene Epshteyn <eepshteyn@nvidia.com>