libzigc: Implement tanh

The changes were tested by running following commands:

```
$ ./build/stage3/bin/zig build -p stage4 -Denable-llvm -Dno-lib

$ stage4/bin/zig build test-libc -Dlibc-test-path=<LIBC-TEST-PATH> -Dtest-filter=tanh -fqemu -fwasmtime --summary line
Build Summary: 1289/1289 steps succeeded
```

lib/c/math.zig

@@ -63,6 +63,7 @@ comptime {
         symbol(&pow, "pow");
         symbol(&pow10, "pow10");
         symbol(&pow10f, "pow10f");
+        symbol(&tanh, "tanh");
     }
 
     if (builtin.target.isMuslLibC()) {
@@ -338,3 +339,7 @@ test "rint" {
     try expectEqual(@as(f64, 2.0), rint(2.5));
     try expectEqual(@as(f64, 4.0), rint(3.5));
 }
+
+fn tanh(x: f64) callconv(.c) f64 {
+    return math.tanh(x);
+}