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=armv8-a

is treated as

$ gcc -march=armv8-a -march=native

Prune joined switches with Negative and RejectNegative to allow
-march=armv8-a to override previous -march=native on command-line.

This is the same fix as was applied for i386 in SVN revision 269164 but for
aarch64 and arm.

2019-09-26  Matt Turner  <mattst88@gmail.com>

	PR driver/69471
	* config/aarch64/aarch64.opt (march=): Add Negative(march=).
	(mtune=): Add Negative(mtune=).
	(mcpu=): Add Negative(mcpu=).
	* config/arm/arm.opt: Likewise.

From-SVN: r276148