Summary:
It currently takes ~32kB of stack trace to run symbolizer, which is
very close to ASan alt stack size (32 kB).

If we exclude `demangle` (which can use unbound stack size in extreme cases),
the heaviest path in symbolizer includes `FrameArray<100>` and three arrays of
`PATH_MAX` (4 kB) size.  This diff removes the former and one of the latters,
reducing this code path from 32 kB to ~10 kB.

Reviewed By: ot, yfeldblum

Differential Revision: D4618467

fbshipit-source-id: e6a53b61b3d5f6e8b892216d2e9b839ed8430d0e