ld: Add --enable-mark-plt configure option

ld: Add --enable-mark-plt configure option

Add --enable-mark-plt linker configure option to mark PLT entries with
DT_X86_64_PLT, DT_X86_64_PLTSZ and DT_X86_64_PLTENT dynamic tags by
default.

	* NEWS: Mention -z mark-plt/-z nomark-plt and --enable-mark-plt.
	* config.in: Regenerated.
	* configure: Likewise.
	* configure.ac: Add --enable-mark-plt.
	(DEFAULT_LD_Z_MARK_PLT): New AC_DEFINE_UNQUOTED.
	* emulparams/x86-64-plt.sh (PARSE_AND_LIST_OPTIONS_X86_64_PLT):
	Support DEFAULT_LD_Z_MARK_PLT.
	* emultempl/elf-x86.em (elf_x86_64_before_parse): New function.
	(LDEMUL_BEFORE_PARSE): New.  Set to elf_x86_64_before_parse for
	x86-64 targets.