[libc][math] Fixed Hypotbf16 build failure. (#186415)

Ref from the build failure in hypotbf16
```CPP
project/libc/src/__support/math/hypotbf16.h:22:33:   required from here
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/__support/FPUtil/Hypot.h:221:44: error: conversion from ‘int’ to ‘StorageType’ {aka ‘short unsigned int’} may change value [-Werror=conversion]
  221 |     r = static_cast<StorageType>((r << 1)) +
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
  222 |         ((tail_bits & current_bit) ? 1 : 0);
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
```

This PR intends to fix that by adding static_cast

NOKEYCHECK=True
GitOrigin-RevId: 63c219b8a866d799de28bb48a23ec32d868c2301