Optimizing: Replace x86 xchg use with xor sequence

On some x86 processors, xchg is serializing even when exchanging two
registers.  Replace the xchgl use with the 3 xor sequence to swap to
registers.  This is generally faster and doesn't serialize the machine.

Change-Id: Iea2cd993d3b70a103bbdd1dbf7818e26ae29387c
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>