Fix libFoundation.so being flagged as requiring executable stack

libFoundation.so is flagged as requiring an executable stack on Linux, although it does not. This is due to GNU as behavior.
Please see discussion at https://bugs.swift.org/browse/SR-2700?jql=text%20~%20%22executable%20stack%22

This was already intended to be solved by #649, but did not work as intended in Ubuntu. Checking for __GNUC__ in addition resolves the issue.

Author: thorsteneb

CoreFoundation/Base.subproj/CFAsmMacros.h

@@ -14,7 +14,7 @@
 #define CONCAT_EXPANDED(a,b) CONCAT(a,b)
 #define _C_LABEL(name) CONCAT_EXPANDED(__USER_LABEL_PREFIX__,name)
 
-#if defined(__GNU__) || defined(__ANDROID__) || defined(__FreeBSD__)
+#if defined(__GNU__) || defined(__GNUC__) || defined(__ANDROID__) || defined(__FreeBSD__)
 #define NO_EXEC_STACK_DIRECTIVE .section .note.GNU-stack,"",%progbits
 #else
 #define NO_EXEC_STACK_DIRECTIVE