package/devel/gdb-arc: Add target GDB for ARC

ARC port of GDB is not yet upstream so we need to use
sources from Synopsys GitHub repo.

Given Synopys' commitment to upstream ARC support in GDB
in the nearest future it might be simpler to add a separate
package for ARC GDB instead of patching generic GDB package.
This way once ARC GDB stuff gets uptreamed we'll only need
to remove that new "gdb-arc" package.

Note 1 very minor change in generic gdb package was done -
it now depends on !arc (while "gdb-arc" depends on "arc").

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>