rootpool/Makefile : support building both x86 and x86_64 versions of the rootpool helper

[jimklimov]

Just in case :-)

$ make
mkdir -p i86pc
gcc -Wall -m32 -o i86pc/rootpool rootpool.c -lzfs -lnvpair
mkdir -p amd64
gcc -Wall -m64 -o amd64/rootpool rootpool.c -lzfs -lnvpair

$ file */*
amd64/rootpool: ELF 64-bit LSB executable, x86-64, version 1, dynamically linked, interpreter /usr/lib/amd64/ld.so.1, not stripped
i86pc/rootpool: ELF 32-bit LSB executable, Intel 80386, version 1, dynamically linked, interpreter /usr/lib/ld.so.1, not stripped