gdb/testsuite: gdb.base/args.exp: use $old_gdbflags last two tests

gdb/testsuite: gdb.base/args.exp: use $old_gdbflags last two tests

All tests in this file append to GDBFLAGS instead of overwriting it,
except the last two.  I noticed because when testing with the
native-extended-remote board, it removes the "set sysroot" argument, and
it causes the test to be very long to run, due to big glibc debug info
being read through the remote target.

I think this oddity is due to a race condition between these two
commits:

  [1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c22261528c50f7760dd6a2e29314662b377eebb4
  [2] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=6b8ce727297b1e40738e50f83a75881b290fe6a6

The first one added the two tests.  The second one changes the test to
append to GDBFLAGS instead of overwriting it.  But the second one was
probably written before the first one was it, so missed the new tests.

Change those two tests to be like the others.

gdb/testsuite/ChangeLog:

	* gdb.base/args.exp: Use $old_gdbflags in all tests.

Change-Id: I531276125ecb70e80f52adbd320ebb85b0c8eba0