amd64/mp_machdep.c: Fix GCC build after r333059

GCC warns about the potentially confusing use of the binary AND ('&')
operator with a left operand containing an addition expression. (The
confusion would be around the operator precedence between the + and & infix
operators.) The warning is converted into an error with -Werror.

No functional change.

This construct was actually introduced in r328083, but r333059 (re)moved the
closing parentheses.

For reference, see http://en.cppreference.com/w/c/language/operator_precedence .