[ARC] Implement compatible function for ARC BFD architectures

[ARC] Implement compatible function for ARC BFD architectures

The general rule for bfd_arch_info_type->compatible (A, B) is that if A and B
are compatible, then this function should return architecture that is more
"feature-rich", that is, can run both A and B.  ARCv2, EM and HS all has same
mach number, so bfd_default_compatible assumes they are the same, and returns
an A.  That causes issues with GDB, because GDB assumes that if machines are
compatible, then "compatible ()" always returns same machine regardless of
argument order.  As a result GDB gets confused because, for example,
compatible(ARCv2, EM) returns ARCv2, but compatible(EM, ARCv2) returns EM,
hence GDB is not sure if they are compatible and prints a warning.

bfd/ChangeLog:

yyyy-mm-dd  Anton Kolesov  Anton.Kolesov@synopsys.com

	 cpu-arc.c (arc_compatible): New function.