aarch64: Give hint for -mcpu options that match -march instead

aarch64: Give hint for -mcpu options that match -march instead

We should redirect users of the erroneous -mcpu=armv8.2-a to use -march instead.
There is an equivalent hint for -march used with a CPU name.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_validate_mcpu): Add hint to use -march
	if the argument matches that.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/spellcheck_11.c: New test.