Fix references to __ehdr_start when it cannot be defined

ld/
	* emultempl/elf32.em (gld${EMULATION_NAME}_before_allocation):
	Don't use bfd_elf_record_link_assignment to mark __ehdr_start
	hidden.  Instead, just do it directly here, and only if it was
	referenced but not defined.

ld/testsuite/
	* ld-elf/ehdr_start-userdef.t: New file.
	* ld-elf/ehdr_start-userdef.d: New file.
	* ld-elf/ehdr_start-strongref.s: New file.
	* ld-elf/ehdr_start-missing.t: New file.
	* ld-elf/ehdr_start-missing.d: New file.
	* ld-elf/ehdr_start-weak.d: New file.
	* ld-mips-elf/ehdr_start-2.nd: Expect __ehdr_start to be global.