OpenBLAS

Fix two mistakes on Arm64 builds

 * Falkor is an ARMv8.0 with ARMv8.1 features, and chosing armv8.1-a for
   march generates instructions it cannot cope with. Reverting it back
   to armv8-a.
 * ThunderX2's build was left with a #define VULCAN, which made it miss
   the right compiler flags in Makefile.arm64, although it did create
   the right library in the end.