OpenBLAS

Support QEMU cpu calling itself 64bit AMD Athlon as well

Some QEMU instances pretend to be "AuthenticAMD" with the same family 6/model 6 even when running on an Intel host
(could be related to qemu or libvirt version and/or kvm availability). Also fix the define to depend on __x86_64__ set by the
compiler, the defines using __64BIT__ will only work for getarch_2nd.