libs/libc: add obstack

[Cynerd]

Summary

Implementation of GlibC's Obstacks (https://www.gnu.org/software/libc/manual/html_node/Obstacks.html) tweaked to be less memory-consuming (although it is way less efficient for a lot of small allocations).

Testing

Tested with a custom application, as well as outside NuttX (except for printf part, of course).