[Bug jit/66594] New: jitted code should use -mtune=native

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66594

            Bug ID: 66594
           Summary: jitted code should use -mtune=native
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: jit
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

We should probably default to -mtune=native when building code in-memory (but
probably to -mtune=generic when compiling to disk?).  Perhaps an option, or
maybe expose these options directly?

Note to self:

  * I see in gcc.c:
      /* As special support for cross compilers we read -mtune=native
             as -mtune=generic.  With native compilers we won't see the
             -mtune=native, as it was changed by the driver.  */
      if (!strcmp (opts->x_ix86_tune_string, "native"))
        {
          opts->x_ix86_tune_string = "generic";
        } 

hence we need to somehow run the *driver* logic for -mtune=native.

Note that that logic can be a no-op under non-bootstrapped builds; see bug
66593.