Add single-precision asin() and acos() to stdlib.

Issue #184.

tests/asin.ispc

@@ -0,0 +1,23 @@
+
+export uniform int width() { return programCount; }
+
+
+bool ok(float x, float ref) { return (abs(x - ref) < 1e-6) || abs((x-ref)/ref) < 1e-6; }
+
+export void f_v(uniform float RET[]) {
+    uniform float vals[8] = { 0, 1, 0.5, -1, -.87, -.25, 1e-3, -.99999999 };
+    uniform float r[8];
+    foreach (i = 0 ... 8)
+        r[i] = sin(asin(vals[i]));
+
+    int errors = 0;
+    for (uniform int i = 0; i < 8; ++i) {
+        if (ok(r[i], vals[i]) == false) {
+            print("error @ %: got %, expected %\n", i, r[i], vals[i]);
+            ++errors;
+        }
+    }
+    RET[programIndex] = errors;
+}
+
+export void result(uniform float RET[]) { RET[programIndex] = 0; }