GDC does not implement DMD style inline assembly, but offers another inline assembly mechanism called extended assembler where the asm block is a template filled out by the compiler and passed to an external assembler executable that understands either AT&T or Intel style on x86.
In the simplest form core.cpuid needs a few versioned asm blocks for GDC that duplicate what the DMD asm blocks do.
Another idea is to replace the accessor properties with direct access to the global constants (mmx, hasPopcnt, etc.) to work around GDC's inability to inline across modules.