Why two arches? – please see the comments
<circle cx="" cy="" r="" />
<path d="
M cx, cy
m -r, 0
a r,r 0 1,0 (r * 2),0
a r,r 0 1,0 -(r * 2),0
"/>
...or a bit shorter (merging the two moves)
<path d="
M cx - r, cy
a r,r 0 1,0 (r * 2),0
a r,r 0 1,0 -(r * 2),0
"/>
Example
<circle cx="100" cy="100" r="75" />
<path d="
M 100, 100
m -75, 0
a 75,75 0 1,0 150,0
a 75,75 0 1,0 -150,0
"/>