PR13616, linker should pad executable sections with nops, not zeros

PR13616, linker should pad executable sections with nops, not zeros

This implements padding of orphan executable sections for PowerPC.
Of course, the simple implementation of bfd_arch_ppc_nop_fill and
removing the NOP definition didn't work, with powerpc64 hitting a
testsuite failure linking to S-records.  That's because the srec
target is BFD_ENDIAN_UNKNOWN so the test of bfd_big_endian (abfd) in
default_data_link_order therefore returned false, resulting in a
little-endian nop pattern.  The rest of the patch fixes that problem
by adding a new field to bfd_link_info that can be used to determine
actual endianness on targets like srec.

	PR 13616
include/
	* bfdlink.h (struct bfd_link_info <big_endian>): New field.
bfd/
	* cpu-powerpc.c (bfd_arch_ppc_nop_fill): New function, use it
	for all ppc arch info.
	* linker.c (default_data_link_order): Pass info->big_endian to
	arch_info->fill function.
ld/
	* emulparams/elf64lppc.sh (NOP): Don't define.
	* emulparams/elf64ppc.sh (NOP): Don't define.
	* ldwrite.c (build_link_order): Use link_info.big_endian.  Move
	code determining endian to use for data_statement to..
	* ldemul.c (after_open_default): ..here.  Set link_info.big_endian.