[libc] Add missing cast in fputil sqrt code

[mcgrathr]

Summary

A cast is necessary to avoid implicit narrowing warnings
when those are enabled.

[abrachet]

Looks like I missed this because I only built for x86. I was meaning to get to it. Thanks

libc/src/__support/FPUtil/generic/sqrt_80_bit_long_double.h

 #include "src/__support/builtin_wrappers.h"
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 namespace fputil {
 namespace x86 {
 
 LIBC_INLINE void normalize(int &exponent, UInt128 &mantissa) {
-  const int shift =
+  const unsigned int shift = static_cast<unsigned int>(
       unsafe_clz(static_cast<uint64_t>(mantissa)) -
-      (8 * sizeof(uint64_t) - 1 - MantissaWidth<long double>::VALUE);
+      (8 * sizeof(uint64_t) - 1 - MantissaWidth<long double>::VALUE));
   exponent -= shift;
   mantissa <<= shift;
 }
 
 // if constexpr statement in sqrt.h still requires x86::sqrt to be declared
 // even when it's not used.
 LIBC_INLINE long double sqrt(long double x);