[Bug target/83743] New: -mcpu=native causes gcc to exit in error if cpu is not recognized

["bergner at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83743

            Bug ID: 83743
           Summary: -mcpu=native causes gcc to exit in error if cpu is not
                    recognized
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bergner at gcc dot gnu.org
  Target Milestone: ---

Running an "old" gcc with "-mcpu=native" that's doesn't have basic support for
the processor it is running on, causes gcc to exit in error.

For example, using an old gcc with -mcpu=native on a POWER9 system gives:

cc1: error: unrecognized argument in option '-mcpu=power9'
cc1: note: valid arguments to '-mcpu=' are: 401 403 405 405fp 440 440fp 464
464fp 476 476fp 505 601 602 603 603e 604 604e 620 630 740 7400 7450 750 801 821
823 8540 8548 860 970 G3 G4 G5 a2 cell e300c2 e300c3 e500mc e500mc64 e5500
e6500 ec603e native power3 power4 power5 power5+ power6 power6x power7 power8
powerpc powerpc64 powerpc64le rs64 titan

My $0.02 worth is that we should still error out, but have a slightly more
useful error message, since the user didn't actually type "-mcpu=power9".