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-generic-1.ll

@@ -864,3 +864,9 @@ declare float @__half_to_float_uniform(i16 %v) nounwind readnone
 declare <WIDTH x float> @__half_to_float_varying(<WIDTH x i16> %v) nounwind readnone
 declare i16 @__float_to_half_uniform(float %v) nounwind readnone
 declare <WIDTH x i16> @__float_to_half_varying(<WIDTH x float> %v) nounwind readnone
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; int8/int16 builtins
+
+define_avgs()
+