non-PIC references to __ehdr_start in pie and shared

Rather than hacking every backend to not discard dynamic relocations
against an undefined hidden __ehdr_start, make it appear to be defined
early.  We want __ehdr_start hidden before size_dynamic_sections so
that it isn't put in .dynsym, but we do need the dynamic relocations
for a PIE or shared library with a non-PIC reference.  Defining it
early is wrong if we don't actually define the symbol later to its
proper value.  (In some cases we want to leave the symbol undefined,
for example, when the ELF header isn't loaded, and we don't have this
infomation available in before_allocation.)

ld/
	* emultempl/elf32.em (gld${EMULATION_NAME}_before_allocation): Define
	__ehdr_start before size_dynamic_sections and restore afterwards.
ld/testsuite/
	* ld-elf/ehdr_start-shared.d: New.
	* ld-elf/ehdr_start-userdef.d: xfail frv.
	* ld-elf/ehdr_start-weak.d: Likewise.
	* ld-elf/ehdr_start.d: Likewise.