Fix imgdiag build rules.

Adjust build rules to avoid building a 64-bit imgdiag
executable on host when HOST_PREFER_32_BIT is true.

This CL addresses an imgdiag related build issue that
appeared on the ART Buildbot after system/core was unpinned
from the master-art manifest
(https://android-review.googlesource.com/#/c/185580), used
by some of our Buildbot configurations:

  ninja: error: 'out/host/linux-x86/obj/SHARED_LIBRARIES/libbacktrace_intermediates/export_includes', needed by 'out/host/linux-x86/obj/EXECUTABLES/imgdiag_intermediates/import_includes', missing and no known rule to make it

Bug: 26051370
Change-Id: I8f3b3c89fb8da0c37a1ffb21b6d835e7ede1e71a