Correct assertion in NIOS2 linker to allow signed 16-buit immediate values.

	PR 19405
	* elf32-nios2.c (nios2_elf32_install_imm16): Allow for signed
	immediate values.
	* elf-eh-frame.c (_bfd_elf_discard_section_eh_frame): Limit the
	number of messages about FDE encoding preventing .eh_frame_hdr
	generation.