Basic: match GCC behaviour for SuS macro

Basic: match GCC behaviour for SuS macro

GCC does not generate `__unix` nor `unix` macros.  The latter already
intrudes into the user's namespace and should be avoided.  Use the
canonical spelling of `__unix__` across all the targets.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294148 91177308-0d34-0410-b5e6-96231b3b80d8