This change adds back off logic in the test and set lock for better contended
lock performance. It uses a simple truncated binary exponential back off
function. The default back off parameters are tuned for x86.

The main back off logic has a two loop structure where each is controlled by a
user-level parameter:
max_backoff - limits the outer loop number of iterations.
    This parameter should be a power of 2.
min_ticks - the inner spin wait loop number of "ticks" which is system
    dependent and should be tuned for your system if you so choose.
    The "ticks" on x86 correspond to the time stamp counter,
    but on other architectures ticks is a timestamp derived
    from gettimeofday().

The user can modify these via the environment variable:
KMP_SPIN_BACKOFF_PARAMS=max_backoff[,min_ticks]
Currently, since the default user lock is a queuing lock,
one would have to also specify KMP_LOCK_KIND=tas to use the test-and-set locks.

Differential Revision: http://reviews.llvm.org/D19020

llvm-svn: 266329