Revert "Move GCStress BacktraceHelper to libbacktrace"

This reverts commit 9491694c36f673149c4eed6918d4a58c328ef103.

Reason for revert: Makes gcstress configs take too long to finish

Change-Id: I2eb03f25d33e9de1d86ca9db74e615fbfc09d51e
Test: None.
3 files changed