Add missing dependency on <machine/_limits.h>.

In r227474, this header file was changed to define SIG_ATOMIC_{MIN,MAX}
in terms of LONG_{MIN,MAX}. Unlike all of the definitions in this header
file, LONG_{MIN,MAX} is provided by <limits.h>. Remove the dependency on
<limits.h> by using __LONG_{MIN,MAX} instead and including
<machine/_limits.h>.

This change is needed to make SIG_ATOMIC_{MIN,MAX} work without
including any other header files.