MinGW64:Target nocona for 64-bit builds [revision]

Revision of !438 (closed)

Per discussion, -march=nocona is only set when X86_64=1. LINUX64=1 does not set this flag.

MINGW64=1 does currently set this flag, but only because we assume x86-64 in src/win32/makefile.cfg. If we stop assuming x86-64 in the future, then X86_64 should not be set by MINGW64 at that time.

M5 is also renamed to ARCHOPTS and can be overridden by command line.