Import obstack.c and obstack.h am: 61a263b27f

Original change: https://android-review.googlesource.com/c/platform/external/obstack/+/1843833

Change-Id: I2e33931d44e8be7bf068eb2d308b6196c9ff73dc
tree: 4bf158aad52e1579720839f76306df099af3144e
  1. include/
  2. libiberty/