[BUILD] make it possible to pass alternative arch at build time

[BUILD] make it possible to pass alternative arch at build time

When trying to build a 32-bit binary on a 64-bit platform, we generally
need to pass "-m32" to gcc, which is not convenient with current makefile.
Note that this option requires gcc >= 3.

In order to ease parameter passing, a new ARCH= makefile option has been
added. If it receives a target architecture, according "-m32"/"-m64" and
"-march=xxxx" will be passed to gcc. Only the generic makefile has been
changed to support this option right now as the need only appeared on Linux.

The spec file now makes use of this option so that rpmbuild can automatically
build with the proper architecture.