Modified some test annotations so that tests on Linux that should be skipped don't show up as XFAIL.

The following two tests showed up as XFAIL even though they should
always be skipped on Linux, due to the @unittest2.expectedFailure
annotation appearing above the @dsym_test annotation:

TestObjCNewSyntax.ObjCNewSyntaxTestCase.test_expr_with_dsym
TestBlocks.BlocksTestCase.test_expr_with_dsym.

For those two, I simply moved the @dsym_test annotation to the top so
that it would be marked for skip ahead of being marked for XFAIL.

TestObjCNewSyntax.ObjCNewSyntaxTestCase.test_expr_with_dwarf I marked
as @skipIfLinux since my understanding is that isn't a valid test to
run on Linux. So rather than categorize as a fail (i.e. something
wrong to be fixed), just skip it. (My recent changes to Linux tests
have been following that model: if it could never work, skip; if it's
broken, mark XFAIL so we can easily track, fix, notice the fix and
adjust accordingly).

TestDeadStrip.DeadStripTestCase.test_with_dwarf I had previously
marked as XFAIL but this would never work on Linux with the current
linker AFAICT.  Marked it as skip.