[committed] Fix minor bug in epiphany port

So I consider this port dead as it semi-randomly fails in reload due to
unrelated changes earlier in the gimple and RTL pipelines.  Regardless Richard
S's late-combine work did show a very obvious error in the port that we should
go ahead and fix as long as the port is in-tree.

The epiphany add-with-immediate instruction allows an 11 bit signed immediate.
That gives the instruction an immediate range of -1024..1023.

The port actually allowed -8192..8191 due to the uber-weird constraint
definition.  I've simplified the constraint to match the hardware documentation
I was able to find.  That was enough to get the epiphany port to build
libgcc/newlib with Richard S's late-combine work.

The testsuite is so flakey on that port (due to the reload failures) that my
tester doesn't run it.  So no comparisons are available.

gcc/
	* config/epiphany/constraints.md (Car): Allow -1024..1023, no more,
	no less.