objcopy --weaken-symbol: apply to STB_GNU_UNIQUE symbols

    PR binutils/28926
    * objcopy.c (filter_symbols): Apply weaken to STB_GNU_UNIQUE symbols
    * NEWS: Mention feature.
    * testsuite/binutils-all/objcopy.exp (objcopy_test_symbol_manipulation): New test.
    * testsuite/binutils-all/weaken-gnu-unique.s: New.