[libc][obvious] fix sqrt when long double is double

[michaelrj]

Summary

Previously, the "fsqrt" instruction was used on all x86_64 platforms
for finding the square root of long doubles. On long double is double
platforms (e.g. windows) this created errors. This patch changes square
root function for long doubles to be the same as the one for doubles if
long doubles are doubles.

libc/src/__support/FPUtil/x86_64/sqrt.h

 }
 
 template <> inline double sqrt<double>(double x) {
   double result;
   __asm__ __volatile__("sqrtsd %x1, %x0" : "=x"(result) : "x"(x));
   return result;
 }
 
+#ifdef LONG_DOUBLE_IS_DOUBLE
+template <> inline long double sqrt<long double>(long double x) {
+  long double result;
+  __asm__ __volatile__("sqrtsd %x1, %x0" : "=x"(result) : "x"(x));
+  return result;
+}
+#else
 template <> inline long double sqrt<long double>(long double x) {
   __asm__ __volatile__("fsqrt" : "+t"(x));
   return x;
 }
+#endif
 
 } // namespace fputil
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_FPUTIL_X86_64_SQRT_H