Cygwin: smallprint.cc: Convert tmpbuf to lockless

Cygwin: smallprint.cc: Convert tmpbuf to lockless

The old technique was from a time when we had to reduce stack pressure
by moving 64K buffers elsewhere.  It was implemented using a static
global buffer, guarded by a muto. However, that adds a lock which may
unnecessarily serialize threads.

Use Windows heap buffers per invocation instead.  HeapAlloc/HeapFree are
pretty fast, scale nicely in multithreaded scenarios and don't serialize
threads unnecessarily.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>