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

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

Add a configure option --enable-relro to decide whether -z relro should
be enabled in ELF linker by default.  Default to yes for all Linux
targets, except FRV, HPPA, IA64 and MIPS, since many relro tests fail
on these targets.

	PR ld/20283
	* NEWS: Mention --enable-relro.
	* configure.ac: Add --enable-relro.
	(DEFAULT_LD_Z_RELRO): New.  Set by --enable-relro.
	* configure.tgt (ac_default_ld_z_relro): Default it to 1 for
	some Linux targets.
	* config.in: Regenerated.
	* configure: Likewise.
	* emultempl/elf32.em (gld${EMULATION_NAME}_before_parse): Set
	link_info.relro to DEFAULT_LD_Z_RELRO.
	* testsuite/config/default.exp (ld_elf_shared_opt): New.
	* testsuite/lib/ld-lib.exp (run_dump_test): Pass
	$ld_elf_shared_opt to ld for ELF targets with shared object
	support.
	(run_ld_link_tests): Likewise.