Bug 480126 - Build failure on Raspberry Pi 5 / OS 6.1.0-rpi7-rpi-v8

Bug 480126 - Build failure on Raspberry Pi 5 / OS 6.1.0-rpi7-rpi-v8

The problem is that the Pi 3+ with raspbian has a 64bit aarch64 OS
but a 32bit gnueabihf userland. If you just run "configure" the
result will be an attempt to use 32-bit GCC with the ARM64 Valgrind
platform, which will fail.

This can be fixed by using the appropriate --host= option.
I've updated README.aarch64 with a description of this.

I've also changed configure.ac to take "arm*" for the
ARM platform (previously it was "armv7*"). That's been
a patch on Debian for a while. It will allow building
on older arm hardware and also armv8 (used on the
Raspberry Pi 3 and later models).