Explicitly specify "signed char" when testing extensions.

The ARM ABIs specify that char is unsigned by default, in contrast to many
others. This test seemed more concerned about the extensions that get applied
to "signed char" than whether that's the default so just add "signed" to the
types.


git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@172716 91177308-0d34-0410-b5e6-96231b3b80d8