compiler_gcc: do not redefine __gnu_attributes

gcc allows extensions to be non compiler specific by defining
__* macros for the attributes supported by gcc. Having a
different definition causes many warnings during the build
(cdefs.h on FreeBSD uses __attribute((__pure__)) where u-boot
uses __attribute__((pure)) for example). Do not redefine
these macros to suppress these warnings.

This patch ignores the checkpatch warning:
WARNING: __packed is preferred over __attribute__((packed))

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>