[Bug 283917] libc failes to include stdint.h with clang>16 and -ffreestanding

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=283917

            Bug ID: 283917
           Summary: libc failes to include stdint.h with clang>16 and
                    -ffreestanding
           Product: Base System
           Version: 14.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: standards
          Assignee: standards@FreeBSD.org
          Reporter: keve@keve.hu

The trivial code
```
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>

int
main(const int argc, const char *argv[])
{
        uint32_t answer = 42;
        printf("%" PRId32 "\n", answer);
        return 0;
}
```

fails to compile with clang17, clang18, clang19, when -ffreestanding is set


```
# env CC=clang19 CFLAGS=-ffreestanding make hello
clang19 -ffreestanding  hello.c  -o hello
In file included from hello.c:2:
/usr/local/llvm19/lib/clang/19/include/stdint.h:261:25: error: typedef
redefinition with different types ('int16_t' (aka 'short') vs '__int_fast16_t'
(aka 'int'))
  261 | typedef __int_least16_t int_fast16_t;
      |                         ^
/usr/include/sys/stdint.h:51:25: note: previous definition is here
   51 | typedef __int_fast16_t          int_fast16_t;
      |                                 ^
In file included from hello.c:2:
/usr/local/llvm19/lib/clang/19/include/stdint.h:262:26: error: typedef
redefinition with different types ('uint16_t' (aka 'unsigned short') vs
'__uint_fast16_t' (aka 'unsigned int'))
  262 | typedef __uint_least16_t uint_fast16_t;
      |                          ^
/usr/include/sys/stdint.h:56:26: note: previous definition is here
   56 | typedef __uint_fast16_t         uint_fast16_t;
      |                                 ^
In file included from hello.c:2:
/usr/local/llvm19/lib/clang/19/include/stdint.h:280:24: error: typedef
redefinition with different types ('int8_t' (aka 'signed char') vs
'__int_fast8_t' (aka 'int'))
  280 | typedef __int_least8_t int_fast8_t;
      |                        ^
/usr/include/sys/stdint.h:50:24: note: previous definition is here
   50 | typedef __int_fast8_t           int_fast8_t;
      |                                 ^
In file included from hello.c:2:
/usr/local/llvm19/lib/clang/19/include/stdint.h:281:25: error: typedef
redefinition with different types ('uint8_t' (aka 'unsigned char') vs
'__uint_fast8_t' (aka 'unsigned int'))
  281 | typedef __uint_least8_t uint_fast8_t;
      |                         ^
/usr/include/sys/stdint.h:55:25: note: previous definition is here
   55 | typedef __uint_fast8_t          uint_fast8_t;
      |                                 ^
4 errors generated.
*** Error code 1
```

Other environments (e.g. Linux, Darwin) compile this code just fine with
-ffreestanding.


A real life project affected by this is e.g. libecc
(https://github.com/libecc/libecc)

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