Support -march=gracemont

Alderlake-N is E-core only, add it as an alias of Alderlake.

gcc/ChangeLog:

	* common/config/i386/cpuinfo.h (get_intel_cpu): Detect
	Alderlake-N.
	* common/config/i386/i386-common.cc (alias_table): Support
	-march=gracemont as an alias of -march=alderlake.