gold: Add a linker configure option --enable-relro

Add a configure option --enable-relro to decide whether -z relro should
be enabled by default.  Default to yes.

	PR ld/20283
	* NEWS: Mention --enable-relro.
	* configure.ac: Add --enable-relro.
	(DEFAULT_LD_Z_RELRO): New.  Set by --enable-relro and default
	to 1.
	* config.in: Regenerated.
	* configure: Likewise.
	* options.h (General_options::relro): Default to
	DEFAULT_LD_Z_RELRO.