bit-rot in target before_parse function

bit-rot in target before_parse function

Copy initialisation over from the elf.em before_parse.  Commit
ba951afb999 2022-05-03 changed behaviour on arm and score regarding
exec stack.  This patch restores the previous behaviour.

	* emultempl/aarch64elf.em (before_parse): Init separate_code,
	warn_execstack, no_warn_rwx_segments and default_execstack.
	* emultempl/armelf.em (before_parse): Likewise.
	* emultempl/scoreelf.em (before_parse): Likewise.
	* testsuite/ld-elf/elf.exp (target_defaults_to_execstack): Return
	true for arm and nacl.