Process the -fno-signed-zeros optimization flag (PR20870)

The driver currently accepts but ignores the -fno-signed-zeros flag. 
This patch passes the flag through and enables 'nsz' fast-math-flag 
generation in IR.

The existing OpenCL flag for the same functionality is made into an
alias here. It may be removed in a subsequent patch.

This should resolve bug 20870 ( http://llvm.org/bugs/show_bug.cgi?id=20870 );
patches for the optimizer were checked in at:
http://llvm.org/viewvc/llvm-project?view=revision&revision=225050
http://llvm.org/viewvc/llvm-project?view=revision&revision=224583

Differential Revision: http://reviews.llvm.org/D6873

llvm-svn: 226915