[AVR][MC] Add ELF flag 'EF_AVR_LINKRELAX_PREPARED' to OBJ files

This is in accordance with avr-gcc, even '-mno-relax' is specified
to avr-gcc, this flag will also be added to the output relocatables.

With this flag set, the GNU ld will perform long call -> short call
optimization for AVR, otherwise not.

Fixes https://github.com/llvm/llvm-project/issues/54508

Reviewed By: MaskRay, jacquesguan, aykevl

Differential Revision: https://reviews.llvm.org/D144617