Lazily initialize fast_sqrt() and pass an Isolate parameter to it

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index 97013debfb0779bf961d298a39266f09b1846c4c..5bce8bb158cb2b34b963fb00022be437442644f4 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -1137,7 +1137,7 @@ inline int NumberOfBitsSet(uint32_t x) {
 bool EvalComparison(Token::Value op, double op1, double op2);
 
 // Computes pow(x, y) with the special cases in the spec for Math.pow.
-double power_helper(double x, double y);
+double power_helper(Isolate* isolate, double x, double y);
 double power_double_int(double x, int y);
 double power_double_double(double x, double y);