When multiple threads call atexit at the same time, some operations must be serialized.  A mutex is used to protect the critical regions.
When multiple threads call atexit at the same time, some operations must
be serialized.  A mutex is used to protect the critical regions.

sbrk() and brk() are not thread safe.  Replace use of sbrk() with
a call to malloc to avoid race when one thread calls atexit
while another thread calls malloc.

Reviewed by:	deischen

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