Enable building with -mthumb for ARM by default

The Diablo autobuilder passes an invalid DEB_BUILD_OPTIONS (options
should be space-separated, not comma-separated), and the Fremantle
autobuilder doesn't pass "thumb" in DEB_BUILD_OPTIONS at all, so the
existing code was an effective no-op.  We know we want Thumb
instructions by default, so build with -mthumb by default and offer a
"nothumb" option to turn it off.