stacks: match exactly before matching nonexactly.

`pid zygote` will match zygote64 as well, but someone using `stacks
zygote` clearly wants the 32-bit version. Attempt an exact match before
we try a nonexact one.

Test: stacks zygote
Test: stacks zygote64
Change-Id: I5fd75cdbe4982c7ea831749c9ba823a2451e5490