Fix strict mode errors in built-ins/Number

test/built-ins/Number/MAX_VALUE/S15.7.3.2_A2.js

test/built-ins/Number/MIN_VALUE/S15.7.3.3_A2.js

test/built-ins/Number/NEGATIVE_INFINITY/S15.7.3.5_A2.js

test/built-ins/Number/NaN/S15.7.3.4_A2.js

test/built-ins/Number/POSITIVE_INFINITY/S15.7.3.6_A2.js

test/built-ins/Number/prototype/S15.7.3.1_A1_T1.js

test/built-ins/Number/prototype/S15.7.3.1_A1_T3.js