AArch64: Have -mcpu=native and -march=native enable extensions when CPU is unknown

Currently when using -mcpu=native or -march=native on a CPU that is unknown to
the compiler the compiler currently just used -march=armv8-a and enables none
of the extensions.

To make this a bit more useful this patch changes it to still use -march=armv8.a
but to enable the extensions.  We still cannot do tuning but at least if using
this on a future SVE core the compiler will at the very least enable SVE etc.

gcc/ChangeLog:

	* config/aarch64/driver-aarch64.c (DEFAULT_ARCH): New.
	(host_detect_local_cpu): Use it.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/cpunative/info_16: New test.
	* gcc.target/aarch64/cpunative/info_17: New test.
	* gcc.target/aarch64/cpunative/native_cpu_16.c: New test.
	* gcc.target/aarch64/cpunative/native_cpu_17.c: New test.