[Bug fortran/81253] New: Flags -march and -mtune are ignored

["kidon at posteo dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81253

            Bug ID: 81253
           Summary: Flags -march and -mtune are ignored
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kidon at posteo dot de
  Target Milestone: ---

I have built GCC 6.3.0 on a Intel Broadwell machine with FreeBSD 11. The binary
package of GCC is used on different client machines (each != Broadwell). Trying
to compile arbitrary Fortran code with GNU Fortran results in an internal
compiler error:

$ gfortran6 test.f08 -o test
<built-in>: internal compiler error: Illegal instruction

It seems that Broadwell-specific instructions are compiled into GNU Fortran
even if additional flags to prevend this are set:

CFLAGS="-march=x86-64 -mtune=generic"
MAKE_ENV+= BOOT_CFLAGS="-march=x86-64 -mtune=generic"
CONFIGURE_ENV+= BOOT_CFLAGS="-march=x86-64 -mtune=generic"

These flags are ignored after stage 3 of the build process. The same behaviour
can be seen when building GCC 4.9/5.4/7.0.