boot: use -march=i386 for both i386 and amd64 builds

boot: use -march=i386 for both i386 and amd64 builds

.. so that consistent compilation algorithms are used for both
architectures as in practice the binaries are expected to be
interchangeable (for time being).
Previously i386 used default setting which were equivalent to
-march=i486 -mtune=generic.
The only difference is using smaller but slower "leave" instructions.

Discussed with:	jhb, dim
MFC after:	29 days