AArch64: Add support for --with-tune

AArch64: Add support for --with-tune

Add support for --with-tune. Like --with-cpu and --with-arch, the argument is
validated and transformed into a -mtune option to be processed like any other
command-line option.  --with-tune has no effect if a -mcpu or -mtune option
is used. The validating code didn't allow --with-cpu=native, so explicitly
allow that.

Co-authored-by:  Delia Burduv  <delia.burduv@arm.com>
Bootstrap OK, regress pass, OK to commit?

2020-09-03  Wilco Dijkstra  <wdijkstr@arm.com>

gcc/
	* config.gcc (aarch64*-*-*): Add --with-tune. Support --with-cpu=native.
	* config/aarch64/aarch64.h (OPTION_DEFAULT_SPECS): Add --with-tune.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_tune_cortex_a76): New
	effective target test.
	* gcc.target/aarch64/with-tune-config.c: New test.
	* gcc.target/aarch64/with-tune-march.c: Likewise.
	* gcc.target/aarch64/with-tune-mcpu.c: Likewise.
	* gcc.target/aarch64/with-tune-mtune.c: Likewise.