Replace GNUPG_GCC_A_ macros by GPGRT_ATTR_ macros.

* common/util.h: Provide replacement for GPGRT_ATTR_ macros when using
libgpg-error < 1.20.
* common/mischelp.h: Ditto.
* common/types.h: Ditto.

Given that libgpg-error is a dependency of all GnuPG related libraries
it is better to define such macros at only one place instead of having
similar macros at a lot of places. For now we need repalcement
macros, though.