arc: Rework default exception handlers for ARC EM and HS

arc: Rework default exception handlers for ARC EM and HS

Initially crt0.S used a special function, declared as weak as a default
exception handler in interrupt vector table. To let user override individual
handlers, this function had multiple names - one for each IVT entry, which,
however, was terribly confusing for the debugger and user - because it
wasn't clear which symbol will be used as a function name in debugger.
Defining multiple separate functions - one for each handler, would resolve
the mess, but would increase code size of crt0.o.

To clean this up, this patch defines exception handlers as weak symbols as
well, but those are defined as just symbols, not functions, hence there
would be less confusion over what is what. At the same time, users still can
redefine exception handlers symbol by creating functions with respective
names.

libgloss/
2016-05-24  Anton Kolesov  <Anton.Kolesov@synopsys.com>

	* arc/crt0.S: Convert memory_error and friends to non-function
	  symbols.