arc: Rename "arc_gdbarch_features" struct

"arc_gdbarch_features" is a data structure containing information
about the ARC architecture: ISA version, register size, etc.
This name is misleading, because although it carries the phrase
"gdbarch", it has nothing to do with the type/interface in GDB.
Traditionaly, "gdbarch" structures are only used for that purpose.
To rectify this, this patch changes the name to "arc_arch_features".

gdb/ChangeLog:

	* arch/arc.h: Rename "arc_gdbarch_features" to
	"arc_arch_features".
	* arc-tdep.h: Likewise.
	* arc-tdep.c: Likewise.