The Intel manual Vol 2A 3-3 shows that byte registers SPL and greater require a REX prefix byte. The is done with the X86Assembler function oneByteOp8(). Use of that function was omitted when the xchg8_*() functions was added.
Created attachment 307746 [details]
I did verify that all other byte ops use the *Op8() formatter function.
I fixed the spelling in the bug title, including the ChangeLog.
Committed r215618: <http://trac.webkit.org/changeset/215618>