Add avg_{up,down}_int{8,16} routines to stdlib

These compute the average of two given values, rounding up and down,
respectively, if the result isn't exact.  When possible, these are
mapped to target-specific intrinsics (PADD[BW] on IA and VH[R]ADD[US]
on NEON.)

A subsequent commit will add pattern-matching to generate calls to
these intrinsincs when the corresponding patterns are detected in the
IR.)

builtins/target-sse4-x2.ll

@@ -573,3 +573,9 @@ define <8 x double> @__max_varying_double(<8 x double>, <8 x double>) nounwind r
   binary2to8(ret, double, @llvm.x86.sse2.max.pd, %0, %1)
   ret <8 x double> %ret
 }
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; int8/int16 builtins
+
+define_avgs()
+