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 7ea50d52 "compiler_gcc: do not redefine
__gnu_attributes" prevented such redefinitions, but this was undone by commit
fb8ffd7c "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>