Subzero: Add fabs intrinsic support.

crosstest/test_arith_fabs.ll

+declare float @llvm.fabs.f32(float)
+declare double @llvm.fabs.f64(double)
+declare <4 x float> @llvm.fabs.v4f32(<4 x float>)
+
+define float @_Z6myFabsf(float %a) {
+  %x = call float @llvm.fabs.f32(float %a)
+  ret float %x
+}
+
+define double @_Z6myFabsd(double %a) {
+  %x = call double @llvm.fabs.f64(double %a)
+  ret double %x
+}
+
+define <4 x float> @_Z6myFabsDv4_f(<4 x float> %a) {
+  %x = call <4 x float> @llvm.fabs.v4f32(<4 x float> %a)
+  ret <4 x float> %x
+}