Disable --gdb execopt on darwin.

Darwin now supports only lldb, so we would have to make the behavior
of the --gdb option platform-dependent.  (That is, start lldb if
running on darwin.)

Separate tests would probably be needed for this option, so just
not testing it on darwin for now.
Modify declint2.prediff to remove leading ./ from gdb output of PGI debug data.
Quiet execflags/bradc/gdbddash/declint2 output on PrgEnv-pgi

The existing prediff script looks for "gdbShouldBreakHere" but in the fast tests, GDB
doesn't print it out at all.

Under PGI, the breakpoint location is printed out with a leading dot-slash.

I solved both problems by modifying the prediff to look for the runtime output
  declint2.chpl:7: error: halt reached - array index out of bounds: (0)
instead.

Since the point of the test is to show that we can run under GDB, if we get as far as
generating error output, we have run the program.  Done.
Merge pull request #723 from hildeth/quietGDBdarwin

Quiet gdb execopts test on darwin and PGI platforms.