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 7ea50d52849fe8ffa5b5b74c979b60b1045d6fc9 "compiler_gcc: do not redefine
__gnu_attributes" prevented such redefinitions, but this was undone by commit
fb8ffd7cfc68b3dc44e182356a207d784cb30b34 "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>
---
changes since v1:
  As requested by Masahiro, elaborate a bit about what cdefs.h is and how these
  warnings are caused. Only this patch is reposted.
README.clang: update FreeBSD instructions

The mentioned binutils port got removed while the patch was
pending. As Ian pointed out there is another port providing
the binutils for arm now. Update the instructions accordingly.

Cc: ian@FreeBSD.org
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
compiler.h: remove duplicated uninitialized_var

Since clang has a different definition for uninitialized_var
it will complain that it is redefined in include/compiler.h.
Since these are already defined in linux/compiler.h just remove
this instance.

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

---
This is only used in ubifs related code and MAKEALL for
arm is fine (besides the 5 boards with build issues)