bfd, binutils, gas: Remove/mark unused variables

Clang generates a warning on unused (technically, written but not read
thereafter) variables.  By the default configuration (with "-Werror"), it
causes a build failure (unless "--disable-werror" is specified).

This commit adds ATTRIBUTE_UNUSED attribute to some of them, which means
they are *possibly* unused (can be used but no warnings occur when
unused) and removes others.

bfd/ChangeLog:

	* elf32-lm32.c (lm32_elf_size_dynamic_sections): Mark unused
	rgot_count variable.
	* elf32-nds32.c (elf32_nds32_unify_relax_group): Remove unused
	count variable.
	* mmo.c (mmo_scan): Mark unused lineno variable.

binutils/ChangeLog:

	* windmc.c (write_rc): Remove unused i variable.

gas/ChangeLog:

	* config/tc-riscv.c (riscv_ip): Remove unused argnum variable.

ld/ChangeLog:

	* pe-dll.c (generate_reloc): Remove unused bi and page_count
	variables.