Fix strict mode errors in built-ins/Function

test/built-ins/Function/S15.3.5_A3_T1.js

test/built-ins/Function/S15.3.5_A3_T2.js

test/built-ins/Function/length/S15.3.5.1_A1_T1.js

test/built-ins/Function/length/S15.3.5.1_A1_T2.js

test/built-ins/Function/length/S15.3.5.1_A1_T3.js

test/built-ins/Function/length/S15.3.5.1_A2_T1.js

test/built-ins/Function/length/S15.3.5.1_A2_T2.js

test/built-ins/Function/length/S15.3.5.1_A2_T3.js

test/built-ins/Function/length/S15.3.5.1_A3_T1.js

test/built-ins/Function/length/S15.3.5.1_A3_T2.js

test/built-ins/Function/length/S15.3.5.1_A3_T3.js

test/built-ins/Function/length/S15.3.5.1_A4_T1.js

test/built-ins/Function/length/S15.3.5.1_A4_T2.js

test/built-ins/Function/length/S15.3.5.1_A4_T3.js

test/built-ins/Function/prototype/S15.3.3.1_A1.js

test/built-ins/Function/prototype/S15.3.3.1_A2.js

test/built-ins/Function/prototype/S15.3.3.1_A3.js

test/built-ins/Function/prototype/S15.3.4_A5.js

test/built-ins/Function/prototype/S15.3.5.2_A1_T1.js

test/built-ins/Function/prototype/S15.3.5.2_A1_T2.js