stdlib: Fix stdbit.h with -Wconversion for older gcc

stdlib: Fix stdbit.h with -Wconversion for older gcc

With gcc 6.5.0, 7.5.0, 8.5.0, and 9.5.0 the tst-stdbit-Wconversion
issues the warnings:

../stdlib/stdbit.h: In function ‘__clo16_inline’:
../stdlib/stdbit.h:128:26: error: conversion to ‘uint16_t {aka short
unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
   return __clz16_inline (~__x);
                          ^
../stdlib/stdbit.h: In function ‘__clo8_inline’:
../stdlib/stdbit.h:134:25: error: conversion to ‘uint8_t {aka unsigned
char}’ from ‘int’ may alter its value [-Werror=conversion]
   return __clz8_inline (~__x);
                         ^
../stdlib/stdbit.h: In function ‘__cto16_inline’:
../stdlib/stdbit.h:232:26: error: conversion to ‘uint16_t {aka short
unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
   return __ctz16_inline (~__x);
                          ^
../stdlib/stdbit.h: In function ‘__cto8_inline’:
../stdlib/stdbit.h:238:25: error: conversion to ‘uint8_t {aka unsigned
char}’ from ‘int’ may alter its value [-Werror=conversion]
   return __ctz8_inline (~__x);
                         ^
../stdlib/stdbit.h: In function ‘__bf16_inline’:
../stdlib/stdbit.h:701:23: error: conversion to ‘uint16_t {aka short
unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
   return __x == 0 ? 0 : ((uint16_t) 1) << (__bw16_inline (__x) - 1);
          ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../stdlib/stdbit.h: In function ‘__bf8_inline’:
../stdlib/stdbit.h:707:23: error: conversion to ‘uint8_t {aka unsigned
char}’ from ‘int’ may alter its value [-Werror=conversion]
   return __x == 0 ? 0 : ((uint8_t) 1) << (__bw8_inline (__x) - 1);
          ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../stdlib/stdbit.h: In function ‘__bc16_inline’:
../stdlib/stdbit.h:751:59: error: conversion to ‘uint16_t {aka short
unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
   return __x <= 1 ? 1 : ((uint16_t) 2) << (__bw16_inline (__x - 1) -
1);
                                                           ^~~
../stdlib/stdbit.h:751:23: error: conversion to ‘uint16_t {aka short
unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
   return __x <= 1 ? 1 : ((uint16_t) 2) << (__bw16_inline (__x - 1) -
1);
          ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../stdlib/stdbit.h: In function ‘__bc8_inline’:
../stdlib/stdbit.h:757:57: error: conversion to ‘uint8_t {aka unsigned
char}’ from ‘int’ may alter its value [-Werror=conversion]
   return __x <= 1 ? 1 : ((uint8_t) 2) << (__bw8_inline (__x - 1) - 1);
                                                         ^~~
../stdlib/stdbit.h:757:23: error: conversion to ‘uint8_t {aka unsigned
char}’ from ‘int’ may alter its value [-Werror=conversion]
   return __x <= 1 ? 1 : ((uint8_t) 2) << (__bw8_inline (__x - 1) - 1);

It seems to boiler down to __builtin_clz not having a variant for 8 or
16 bits.  Fix it by explicit casting to the expected types.

Checked on x86_64-linux-gnu and i686-linux-gnu with gcc 9.5.0.