pipeline: salsaci should use /usr/bin/setarch when building i386 on x86_64

libdecaf has had build failures for i386 both on the Salsa pipeline[0] and on reproducible-builds.org (but not on buildd.d.o). I traced it down to /usr/share/perl5/Debian/Debhelper/Dh_Lib.pm:is_cross_compiling() which assumes that if $DEB_BUILD_GNU_TYPE == $DEB_HOST_GNU_TYPE then no cross-compiling is being done, and logic in /usr/share/perl5/Debian/Debhelper/Buildsystem/cmake.pm that is necessary for cross-compiling is bypassed which results in CMake doing its own check whether to cross-compile. That check is based on the output of uname -m which here is "x86_64" and which is then also relayed to the build scripts. Since the installed gcc toolchain is for i686, uint128_t does not exist, but thinking the build is for x86_64 the builds scripts assume it does which leads to compiler errors. salsaci should make uname -m return the expected value of i686 by running dpkg-buildpackage under setarch i686.

0: https://salsa.debian.org/pkg-voip-team/linphone-stack/libdecaf/-/jobs/3380313/raw