MINOR: tools: add a function to write a thread execution context.

MINOR: tools: add a function to write a thread execution context.

The new function chunk_append_thread_ctx() appends to a buffer the given
execution context based on its type and pointer. The goal is to easily
use it in profiling output and thread dumps. For now it only handles
TH_EX_CTX_NONE (which prints nothing) and TH_EX_CTX_OTHER (which indicates
"other ctx" followed by the pointer). It will be extended by new types as
they arrive.