re PR target/56010 (Powerpc, -mcpu=native and -mtune=native use the wrong name for...

re PR target/56010 (Powerpc, -mcpu=native and -mtune=native  use the wrong name for target 7450)

	PR target/56010
	PR target/83743
	* config/rs6000/driver-rs6000.c: #include "diagnostic.h".
	#include "opts.h".
	(rs6000_supported_cpu_names): New static variable.
	(linux_cpu_translation_table): Likewise.
	(elf_platform) <cpu>: Define new static variable and use it.
	Translate kernel AT_PLATFORM name to canonical name if needed.
	Error if platform name is unknown.

From-SVN: r257305