[libc] Use __attribute__((__nothrow__)) for __NOEXCEPT in C (#114653)

Consistent with glibc headers, where `noexcept` is used in C++
(or `throw()` in older C++ which llvm-libc doesn't support) in
the public function declarations, `__attribute__((__nothrow__))` is
used in C for compilers that support it.

Author: frobtech

libc/include/__llvm-libc-common.h

@@ -53,7 +53,11 @@
 #define __restrict restrict // C99 and above support the restrict keyword.
 
 #undef __NOEXCEPT
+#ifdef __GNUC__
+#define __NOEXCEPT __attribute__((__nothrow__))
+#else
 #define __NOEXCEPT
+#endif
 
 #endif // __cplusplus