AMD_SEV and other constants used in common code, but only defined for amd64

[Jakob-Naucke]

#18 introduced some constants, e.g. AMD_SEV, in cpuid_amd64.go, and then used them in cpuid.go. They will be undefined when building on non-x86_64 architectures.
e.g. on s390x:

$ go get github.com/intel-go/cpuid
# github.com/intel-go/cpuid
go/pkg/mod/github.com/intel-go/[email protected]/cpuid.go:291:2: undefined: AMD_SME
go/pkg/mod/github.com/intel-go/[email protected]/cpuid.go:292:2: undefined: AMD_SEV
go/pkg/mod/github.com/intel-go/[email protected]/cpuid.go:293:2: undefined: AMD_PAGE_FLUSH_MSR
go/pkg/mod/github.com/intel-go/[email protected]/cpuid.go:294:2: undefined: AMD_SEV_ES

/cc @jimcadden