FIXME for merging of e_flags and .gnu.attributes

FIXME for merging of e_flags and .gnu.attributes

Code in the linker, present before the addition of .gnu.attributes
support, results in shared libraries not being considered by BFD when
merging e_flags and .gnu.attributes from input files to the output.
That doesn't seem correct to me, but I don't know enough about all the
various ABIs to change the behaviour with any confidence.  So this
patch merely punts on dynamic objects in merge_private_bfd_data target
functions, with a FIXME for maintainer attention.

I haven't excluded shared libraries from being considered where the
target merge_private_bfd_data (a) already has code dealing with shared
libraries, or (b) where that function just sets the output to the most
constraining arch/mach combination and other fairly trivial merges, or
(c) when the target has no shared library linker support.

In (a) are: arc, arm, aarch64, riscv, sparc.
In (b) are: bpf, cris, csky, m32r, m68k, mn10300, nios2, tilegx,
	    tilepro, vax, visium, xtensa.
In (c) are: bpf, cr16, h8300, iq2000, m32c, m68hc11, m68hc12, mcore,
	    mep, msp430, mt, rl78, rx, v850.

	PR 25882
	* elf32-bfin.c (elf32_bfin_merge_private_bfd_data): Add FIXME.
	* elf32-frv.c (frv_elf_merge_private_bfd_data): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_merge_private_bfd_data): Likewise.
	* elf32-nds32.c (nds32_elf_merge_private_bfd_data): Likewise.
	* elf32-score.c (s3_elf32_score_merge_private_bfd_data): Likewise.
	* elf32-score7.c (s7_elf32_score_merge_private_bfd_data): Likewise.
	* elf32-sh.c (sh_elf_merge_private_data): Likewise.
	* elf32-tic6x.c (elf32_tic6x_merge_attributes): Likewise.
	* elf64-ia64-vms.c (elf64_ia64_merge_private_bfd_data): Likewise.
	* elfnn-ia64.c (elfNN_ia64_merge_private_bfd_data): Likewise.