[ECOS] Mutex optimization for uncontested cases

[Luoqi Chen]

I have noticed very poor performance of stdio stream under eCos, about 4 times
slower than vxworks. Howerver I was able to improve the performance by a factor
of 2 by disabling the CYGSEM_LIBC_STDIO_THREAD_SAFE_STREAMS option. This points
to an inefficiency in the current mutex implementation that we do not optimize
for the most common cases of uncontested mutex acquisitions. I wonder if eCos
developers are aware of this problem and if there is any effort under way
trying to resolve it.

-lq