compiler_gcc: prevent redefining attributes

The libc headers on FreeBSD and likely related projects as well contain an
header file, cdefs.h which provides similiar functionality as linux/compiler.h.
It provides compiler independent defines like __weak __packed, to allow
compiling with multiple compilers which might have a different syntax for such
extension.

Since that header file is included in multiple standard headers, like stddef.h
and stdarg.h, multiple definitions of those defines will be present if both are
included. When compiling u-boot the compiler will warn about it hundreds of
times since e.g. common.h will include both files indirectly.

commit 7ea50d5284 "compiler_gcc: do not redefine
__gnu_attributes" prevented such redefinitions, but this was undone by commit
fb8ffd7cfc "compiler*.h: sync
include/linux/compiler*.h with Linux 3.16".

Add the checks back where necessary to prevent such warnings.

As the original patch this checkpatch warning is ignored:
"WARNING: Adding new packed members is to be done with care"

Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

include/linux/compiler-gcc.h

@@ -64,8 +64,12 @@
 #endif
 
 #define __deprecated			__attribute__((deprecated))
+#ifndef __packed
 #define __packed			__attribute__((packed))
+#endif
+#ifndef __weak
 #define __weak				__attribute__((weak))
+#endif
 
 /*
  * it doesn't make sense on ARM (currently the only user of __naked) to trace
@@ -91,8 +95,12 @@
  * would be.
  * [...]
  */
+#ifndef __pure
 #define __pure				__attribute__((pure))
+#endif
+#ifndef __aligned
 #define __aligned(x)			__attribute__((aligned(x)))
+#endif
 #define __printf(a, b)			__attribute__((format(printf, a, b)))
 #define __scanf(a, b)			__attribute__((format(scanf, a, b)))
 #define  noinline			__attribute__((noinline))
@@ -115,4 +123,6 @@
  */
 #define uninitialized_var(x) x = x
 
+#ifndef __always_inline
 #define __always_inline		inline __attribute__((always_inline))
+#endif