integer-gmp: improve cross-compiling support GmpDerivedConstants.h

Before the patch GmpDerivedConstants.h was generated
by running mkGmpDerivedConstants on target.

Now it's generated only with help of autoconf macros.
Tested on --target=ia64-unknown-linux-gnu.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>