Quiet gdb execopts test on darwin and PGI platforms.

Darwin no longer supports gdb by default (it uses lldb instead).  There is currently no support for a --lldb runtime flag on Chapel executables, so I added a line to the general SKIPIF to just skip this directory on darwin.

GDB used to print out "Breakpoint 1, gdbShouldBreakHere() ... " when starting up, but it does not do this now with the fast tests, nor on PGI.  So the declint2.chpl test fails.  Since the intent of the test is to show that GDB will actually run the program, I changed the prediff file to just grep for some text that appears in the program output.  Now this test passes on both PGI and gnu platforms.