[PATCH, rs6000] Add vec_nabs builtin support

[Carl E. Love]

GCC maintainers:

The following patch adds two more built-ins that are missing.
Specifically:

        vector signed char vec_nabs (vector signed char)
	vector signed short vec_nabs (vector signed short)
	vector signed int vec_nabs (vector signed int)
	vector signed long long vec_nabs (vector signed long long)
	vector float vec_nabs (vector float)
	vector double vec_nabs (vector double)
         
        
The patch has been boot strapped and tested on
powerpc64le-unknown-linux-gnu (Power 8 LE) and on 
powerpc64-unknown-linux-gnu (Power 8 BE 64-bit, 32-bit) and on 
powerpc64-unknown-linux-gnu (Power 7 64-bit, 32-bit) with no
regressions.

Is this OK for trunk?

                    Carl Love


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