[binutils-gdb][ld][AArch64] Fix group_sections algorithm [PR25665]

[Nick Clifton]

Hi Wilco,

> 2020-04-27  Wilco Dijkstra  <wdijkstr@arm.com>
> 
> 	PR ld/25665
> 	* bfd/elfnn-aarch64.c (group_sections): Copy implementation
> 	from elf32-arm.c.
> 	* testsuite/ld-aarch64/aarch64-elf.exp: Add new test.
> 	* testsuite/ld-aarch64/farcall-group.s: New large group test.
> 	* testsuite/ld-aarch64/farcall-group.d: Likewise.

[nit-pick: The above should really be two changelog entries, one
for the bfd/ directory and one for the ld/directory].

Approved and applied.

Note however that there were a couple of problems with the new test:

> +0000000000400078 <_start>:

This start address only works for aarch64-linux-gnu toolchains,  other
configurations, eg aarch64-elf, use a different default start address.

> + 440007c:	d503201f 	.word	0xd503201f

This disassembly only works for little-endian aarch64 configurations.
If you configure a big-endian toolchain then they are byte swapped.

> + 4400088:	90fe0010 	adrp	x16, 400000 <_start\-0x78>

Another aarch64-linux-gnu dependency.  In bare metal toolchains this
offset is displayed as "_stack+0x378000"

I fixed these problems, but in the future it would be great if you could
check with more than one configuration of the aarch64 binutils.

Cheers
  Nick