Fix __isnanl on aarch64.

This fixes the math.__isnanl, math.powl, and math.fdiml tests.

Change-Id: I0086e4a1a82d0e2708fa07cf580a6d9778c548f2
diff --git a/libm/fake_long_double.c b/libm/fake_long_double.c
index 751911a..756e35b 100644
--- a/libm/fake_long_double.c
+++ b/libm/fake_long_double.c
@@ -31,7 +31,7 @@
 int __fpclassifyl(long double a1) { return __fpclassifyd(a1); }
 int __isfinitel(long double a1) { return __isfinite(a1); }
 int __isinfl(long double a1) { return __isinf(a1); }
-int __isnanl(long double a1) { return isnan(a1); }
+int __isnanl(long double a1) { return (isnan)(a1); }
 int __isnormall(long double a1) { return __isnormal(a1); }
 int __signbitl(long double a1) { return __signbit(a1); }