Disable 32bit builds

Blender upstream no longer supports[1][2] 32bit builds. Such a build was removed[3] from debian already, but d/control still doesn't exclude, say, armhf.

Furthermore, the current version in mantic is failing[4] to build on armhf:

/<<PKGBUILDDIR>>/source/blender/blenlib/intern/string.c: In function ‘BLI_str_format_int_grouped’:
/<<PKGBUILDDIR>>/source/blender/blenlib/intern/string.c:1128:23: error: conversion to ‘int’ from ‘size_t’ {aka ‘unsigned int’} may change the sign of the result [-Werror=sign-conversion]
 1128 | const int num_len = BLI_snprintf(src, sizeof(src), "%d", num);

(and more, similar, errors)

It's gone from debian stable already:
$ rmadison -udebian blender -aarmhf
blender | 2.79.b+dfsg0-7+deb10u1 | oldoldstable | armhf
blender | 2.82.a+dfsg-1~bpo10+1 | buster-backports | armhf
blender | 2.83.5+dfsg-5+deb11u1 | oldstable | armhf

1. https://archive.blender.org/lists/bf-committers/2019-August/050124.html
2. https://projects.blender.org/blender/blender/issues/67184
3. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1024626
4. https://launchpad.net/ubuntu/+source/blender/3.4.1+dfsg-2ubuntu1/+build/26480650