[builtins] Fix a check from __GNU__ to __GNUC__ for disabling executable stack.

[manojgupta]

Summary

Neither GCC nor Clang define GNU. Instead use GNUC
for the check.

[manojgupta]

Tested that GCC and Clang do not define

__GNU__

$ cat blah.c

#ifdef __GNU__
aabb // Should error out if __GNU__ is defined
#endif

$ gcc -c blah.c No error
$ bin/clang -c blah.c No error

[echristo]

LGTM.

[krytarowski]

__GNU__ is for GNU/Hurd not, GNU compiler, please revert

We don't want this on NetBSD.

compiler-rt/trunk/lib/builtins/assembly.h

 #define FILE_LEVEL_DIRECTIVE
 #if defined(__arm__)
 #define SYMBOL_IS_FUNC(name) .type name,%function
 #else
 #define SYMBOL_IS_FUNC(name) .type name,@function
 #endif
 #define CONST_SECTION .section .rodata
 
-#if defined(__GNU__) || defined(__ANDROID__) || defined(__FreeBSD__)
+#if defined(__GNUC__) || defined(__ANDROID__) || defined(__FreeBSD__)
 #define NO_EXEC_STACK_DIRECTIVE .section .note.GNU-stack,"",%progbits
 #else
 #define NO_EXEC_STACK_DIRECTIVE
 #endif
 
 #else // !__APPLE__ && !__ELF__
 
 #define HIDDEN(name)