is_underscore_target for ld-elf tests

is_underscore_target for ld-elf tests

and correct targ_underscore in config.bfd

bfd/
	* config.bfd: Correct targ_underscore for epiphany, ip2k,
	m32c, mn10200, pru, rl78, rx, crisv32 and v850.
ld/
	* testsuite/lib/ld-lib.exp (is_underscore_target): New.
	* testsuite/ld-elf/elf.exp (ASFLAGS): Define UNDERSCORE.
	* testsuite/ld-elf/pr21562a.s: If UNDERSCORE defined,
	reference sym with prefix.
	* testsuite/ld-elf/pr21562b.s: Likewise.
	* testsuite/ld-elf/sizeof.s: Likewise.
	* testsuite/ld-elf/startof.s: Likewise.
	* testsuite/ld-elf/pr14156a.d: Adjust for extra symbols.
	* testsuite/ld-elf/pr21562a.d: Remove underscore target from
	xfails, and match prefixed symbol.
	* testsuite/ld-elf/pr21562b.d: Likewise.
	* testsuite/ld-elf/pr21562c.d: Likewise.
	* testsuite/ld-elf/pr21562d.d: Likewise.
	* testsuite/ld-elf/pr21562e.d: Likewise.
	* testsuite/ld-elf/pr21562f.d: Likewise.
	* testsuite/ld-elf/pr21562g.d: Likewise.
	* testsuite/ld-elf/pr21562h.d: Likewise.
	* testsuite/ld-elf/pr21562i.d: Likewise.
	* testsuite/ld-elf/pr21562j.d: Likewise.
	* testsuite/ld-elf/pr21562k.d: Likewise.
	* testsuite/ld-elf/pr21562l.d: Likewise.
	* testsuite/ld-elf/pr21562m.d: Likewise.
	* testsuite/ld-elf/pr21562n.d: Likewise.
	* testsuite/ld-elf/sizeofa.d: Likewise.
	* testsuite/ld-elf/sizeofb.d: Likewise.
	* testsuite/ld-elf/sizeofc.d: Likewise.
	* testsuite/ld-elf/startofa.d: Likewise.
	* testsuite/ld-elf/startofb.d: Likewise.
	* testsuite/ld-elf/startofc.d: Likewise.