When -march=native is passed to host_detect_local_cpu to the backend,
it overrides all command lines after it.  That means

$ gcc -march=native -march=skylake-avx512

is the treated as

$ gcc -march=skylake-avx512 -march=native

Prune joined switches with Negative and RejectNegative to allow
-march=skylake-avx512 to override previous -march=native on command-line.

gcc/

	PR driver/69471
	* opts-common.c (prune_options): Also prune joined switches
	with Negative and RejectNegative.
	* config/i386/i386.opt (march=): Add Negative(march=).
	(mtune=): Add Negative(mtune=).
	* doc/options.texi: Document Negative used together with Joined
	and RejectNegative.

gcc/testsuite/

	PR driver/69471
	* gcc.dg/pr69471-1.c: New test.
	* gcc.dg/pr69471-2.c: Likewise.
	* gcc.target/i386/pr69471-3.c: Likewise.

From-SVN: r269164