[v3] mutex, condition_variable

[Benjamin Kosnik <bkoz at redhat dot com>]

This adds in preliminary support for C++0x mutex/condition_variable, as
first posted here:
http://gcc.gnu.org/ml/libstdc++/2008-03/msg00059.html

Large parts of mutex are taken from the existing support in
ext/concurrence.h, which explains the copyright dates on that file. 

This is the the subset of chapter 30 that (arguably) will not require
new extensions to the gthr.h interface, and so my thinking was to stage
this in first. 

tested x86_64/linux
tested x86_64/linux --disable-threads

-benjamin

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