create_object_file doesn't set correct ELF header flags for AVR targets

[agausmann]

(Downstream issue: Rahix/avr-hal#388)

A bug was introduced in #103240 (5642a75) when compiling for AVR targets, causing errors in linking which look like:

error: linking with `avr-gcc` failed: exit status: 1
  |
  = note: "avr-gcc" "-mmcu=attiny404" "/tmp/rustcmivxhx/symbols.o" "/home/goose/dev/hwlife/life-cell/firmware/target/avr-attiny404/debug/deps/life_cell-6668cd99ba2eaeff.avrxmega_hal-cc404b4688795bef.avrxmega_hal.d6125529-cgu.0.rcgu.o.rcgu.o" "-Wl,--as-needed" "-L" "/home/goose/dev/hwlife/life-cell/firmware/target/avr-attiny404/debug/deps" "-L" "/home/goose/dev/hwlife/life-cell/firmware/target/debug/deps" "-L" "/home/goose/build/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/avr-attiny404/lib" "-Wl,-Bstatic" "/home/goose/dev/hwlife/life-cell/firmware/target/avr-attiny404/debug/deps/libcompiler_builtins-dfbc3c77452aecc4.rlib" "-Wl,-Bdynamic" "-lgcc" "-Wl,-znoexecstack" "-L" "/home/goose/build/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/avr-attiny404/lib" "-o" "/home/goose/dev/hwlife/life-cell/firmware/target/avr-attiny404/debug/deps/life_cell-6668cd99ba2eaeff.elf" "-Wl,--gc-sections" "-no-pie"
  = note: /usr/lib/arduino/hardware/tools/avr/bin/../lib/gcc/avr/7.3.0/../../../../avr/bin/ld: avr architecture of input file `/tmp/rustcmivxhx/symbols.o' is incompatible with avr:103 output
          collect2: error: ld returned 1 exit status

The PR in question added support for AVR architecture in create_object_file, but it doesn't actually make the necessary changes to set the e_flags field, which is used to specify the AVR ISA revision. As a knock-on-effect, this causes add_linked_symbol_object to avoid an early return and succeed in creating a symbols.o file, but this object file doesn't have e_flags set correctly.

I believe that is the cause of this error; the AVR linker checks the e_flags field to make sure the ISA revision matches what is expected, and rustc is not setting that field correctly.

[agausmann]

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