Replace direct PSTL uses of assert() with a macro

Replace direct PSTL uses of assert() with a macro

This also replaces calls to __TBB_ASSERT so that there are two macro
definitions provided by c++config -
	    __PSTL_ASSERT(_Condition)
	    __PSTL_ASSERT_MSG(_Condition, _Message)

	    * include/bits/c++config:
	    Add definition for __PSTL_ASSERT.
	    Add definition for __PSTL_ASSERT_MSG.
	    * include/pstl/algorithm_impl.h: Replace use of assert().
	    * include/pstl/numeric_impl.h: Replace use of assert().
	    * include/pstl/parallel_backend_tbb.h:
	    Replace use of assert().
	    Replace use of __TBB_ASSERT().

	    * include/pstl/parallel_backend_utils.h: Replace use of assert().

From-SVN: r270293