[AArch64] Use move-if-change for aarch64-tune.md

[AArch64] Use move-if-change for aarch64-tune.md

If aarch64-tune.md was older than gentune.sh or aarch64-cores.def,
we'd try to overwrite it even if the current contents were correct.
This could cause problems with read-only source directories and
could cause spurious copying for rsync --archive.

2020-01-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/t-aarch64 ($(srcdir)/config/aarch64/aarch64-tune.md):
	Depend on...
	(s-aarch64-tune-md): ...this new stamp file.  Pipe the new contents
	to a temporary file and use move-if-change to update the real
	file where necessary.

From-SVN: r279900