Implement some missing maths-related intrinsics

Author: tomaka

src/library.js

@@ -1398,10 +1398,14 @@ LibraryManager.library = {
   llvm_sqrt_f64: 'Math_sqrt',
   llvm_pow_f32: 'Math_pow',
   llvm_pow_f64: 'Math_pow',
+  llvm_powi_f32: 'Math_pow',
+  llvm_powi_f64: 'Math_pow',
   llvm_log_f32: 'Math_log',
   llvm_log_f64: 'Math_log',
   llvm_exp_f32: 'Math_exp',
   llvm_exp_f64: 'Math_exp',
+  llvm_trunc_f64: 'Math_trunc',
+  llvm_floor_f64: 'Math_floor',
 
   round__asm: true,
   round__sig: 'dd',

src/preamble.js

@@ -1764,6 +1764,7 @@ var Math_imul = Math.imul;
 var Math_fround = Math.fround;
 var Math_min = Math.min;
 var Math_clz32 = Math.clz32;
+var Math_trunc = Math.trunc;
 
 // A counter of dependencies for calling run(). If we need to
 // do asynchronous work before running, increment this and