test: More portable use of mmap(MAP_ANON)

test: More portable use of mmap(MAP_ANON)

From the Linux manual:
  MAP_ANONYMOUS
     The  mapping  is  not  backed  by any file; its contents
     are initialized to zero.  The fd and offset arguments are
     ignored; however, some implementations require fd to be -1
     if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable
     applications  should  ensure  this.

FreeBSD is such a case, it wil just return an error.

 - Used in:
     src/test/on_exit.cc
     src/test/system/cross_process_sem.cc

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>