linux-user/x86_64: Allocate vsyscall page as a commpage

We're about to start validating PAGE_EXEC, which means that we've
got to mark the vsyscall page executable.  We had been special
casing this entirely within translate.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>