our_cpuid function losing 3rd parameter in 64-bit Unix.

Ran into this when trying to read AVX-512 feature bits (xref #1312 )

        DECLARE_FUNC(our_cpuid)
GLOBAL_LABEL(our_cpuid:)
        mov      REG_XAX, ARG1
        mov      REG_XDX, ARG2
        mov      REG_XCX, ARG3

In Unix, ARG3 == REG_XDX.

Apparently this didn't matter so far, but it does when trying to read extended feature bits with eax = 7, ecx = 0.