Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import libgcroots version 0.2.1.

This library abstracts architecture-dependent part of garbage collector
roots acquisition such as register windows of SPARC and register stack
backing store of IA-64. Main part of the code is based on Boehm GC 7.0.

With this library, you can easily write your own garbage collector such as
for small footprint, some application-specific optimizations, just learning
or to test experimental ideas.