Build 32-bit libstand under boot/

We need a 32-bit libstand on 64-bit platforms for the benefit of various
bootloaders.  We previously built only this 32-bit version, installed as
/usr/lib/libstand.a.

A 64-bit libstand consumer will arrive in the near future, so move the
bootloader-specific 32-bit version to boot/libstand32.

Explicitly link against this version in the 32-bit loaders.