compiler: move LLVM math builtin support into the compiler

This simplifies src/runtime/math.go, which I eventually want to remove
entirely by moving the given functionality into the compiler.

compiler/compiler_test.go

@@ -48,6 +48,8 @@ func TestCompiler(t *testing.T) {
 		{"pragma.go", ""},
 		{"goroutine.go", "wasm"},
 		{"goroutine.go", "cortex-m-qemu"},
+		{"intrinsics.go", "cortex-m-qemu"},
+		{"intrinsics.go", "wasm"},
 	}
 
 	for _, tc := range tests {