stdatomic: make atomics compatible with GCC-14

https://gcc.gnu.org/pipermail/gcc-patches/2023-September/631525.html
brings c_atomic and cxx_atomic definitions into GCC.
This patch makes atomics type detection correct for GCC.

freebsd/freebsd-src@680f40f

Author: Lapshin

newlib/libc/include/stdatomic.h

@@ -43,7 +43,8 @@
 #include <stdint.h>
 #endif
 
-#if __has_extension(c_atomic) || __has_extension(cxx_atomic)
+#if (__has_extension(c_atomic) || __has_extension(cxx_atomic)) && \
+    defined(__clang__)
 #define	__CLANG_ATOMICS
 #elif __GNUC_PREREQ__(4, 7)
 #define	__GNUC_ATOMICS