[Bug ld/31124] New: AVR: Support new Emulations for devices with FLMAP

[gjl at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31124

            Bug ID: 31124
           Summary: AVR: Support new Emulations for devices with FLMAP
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: gjl at gcc dot gnu.org
  Target Milestone: ---

Devices from the AVR64* and AVR128* families (from avrxmega2 and avrxmega4)
see a 32 KiB portion of their program memory in the RAM address space.

Which 32 KiB segment is visible is determined by the bit-field
NVMCTRL_CTRLB.FLMAP.

This can be used to place .rodata in flash (it's currently located in RAM like
for all other AVR devices, except the ones from avrtiny and avrxmega3).

* The user should be able to chose the old .rodata location by means of
  a command line option like, say -mrodata-in-ram.  This is needed
  to return to the current behaviour with rodata in RAM if desired.

* The user may chose which 32 KiB block holds the rodata section.

* Provide symbols so that the startup code from AVR-LibC is able to set
  up NVMCTRL_CTRLB.FLMAP that controls which 32 KiB block is visible in
  the RAM address space.

* New (default) linker description files are needed.  This requires new
  emulations like avrxmega2_flmap and avrxmega4_flmap.

* The startup-code from AVR-LibC should work irrespective of the chosen
  emulation, so that we retain the one-crt-per-device policy for simplicity.

* In all cases, the default configurations should work correctly without any
  user interventions / special code, irrespective of -m[no-]rodata-in-ram.

* The multilib structure is unchanged.

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