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.