[Bug target/64784] New: -march=native should be supported

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64784

            Bug ID: 64784
           Summary: -march=native should be supported
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Target: aarch64

Like -mcpu=native, we should support -march=native too.  This selects the
native arch (armv8 or armv8.1) plus turns on the extensions which are supported
like ASE, CRC, etc.