c-auto-align-backslashes

c-auto-align-backslashes is a variable defined in `cc-vars.el'.
Its value is

t

Documentation:
*Align automatically inserted line continuation backslashes.
When line continuation backslashes are inserted automatically for line
breaks in multiline macros, e.g. by M-x c-context-line-break, they are
aligned with the other backslashes in the same macro if this flag is
set. Otherwise the inserted backslashes are preceded by a single
space.

You can customize this variable.