argp: Do not override GCC keywords with macros [BZ #16907]

glibc provides fallback definitions already.  It is not necessary to
suppress warnings for unknown attributes because GCC does this
automatically for system headers.

This commit does not sync with gnulib because gnulib has started to use
_GL_* macros in the header file, which are arguably in the gnulib
implementation space and not suitable for an installed glibc header
file.

(cherry picked from commit 2c820533)