bug#43945: Support thumbv7a architecture

[KOLANICH via Bug reports for Automake]

Hello. When I crosscompiled some app I had to patch some file generated by 
autoreconf -i, where I have to add thumbv7[arm] (using the analogy to 
armv7[arm] already present there) there. Though I haven't found the source file 
corresponding to it, so no patches. Feel free to fix yourselves.