Add new testcase testing some corner-cases for expm1.  Provided by Matthew Brett


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6593 4407c894-4637-0410-b4f5-ada5f102cad1
Fix corner-case expm1(-0) == -0.  Found by Matthew Brett.


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6594 4407c894-4637-0410-b4f5-ada5f102cad1