Return 0-values for XGETBV64 and CPUID64 when CRYPTOPP_DISABLE_ASM is in effect (GH #1240)

Some folks were defining CRYPTOPP_DISABLE_ASM and not building the *.asm files on WIndows. That happened to work until we refactored code for XGetBV and CpuId.

These alternate build systems are going to be the death of us...