modify SAVE_XMM for potential 64bit use

the win64 abi requires saving and restoring xmm6:xmm15. currently
SAVE_XMM and RESTORE XMM only allow for saving xmm6:xmm7. allow
specifying the highest register used and if the stack is unaligned.

Change-Id: Ica5699622ffe3346d3a486f48eef0206c51cf867