Simplify x86-64 addressing mode syntax.

Simplify x86-64 addressing mode syntax.

*** NOTE:  Since this changes machines/x86-64/insmac.scm, you must
resyntax the compiler afresh, e.g. by running

scheme --band runtime.com <<EOF
(for-each load-option '(SF CREF))
(load "compiler.sf")
EOF

Otherwise, the compiler will expand machines/x86-64/insutl.scm using
the old macro definitions, not the new ones, and grow very confused.
Using the host compiler's macros is pretty bogus.