npx: Fix XMM register count issue in 'savexmm64' struct

npx: Fix XMM register count issue in 'savexmm64' struct

On 64-bit systems, there are 16 XMM registers (XMM0 - XMM15).  And
FXSAVE64/FXRSTOR64 instructions save/restore them to/from the 512-byte
data area.

So fix the 'savexmm64' structure to say 'struct xmmacc sv_xmm[16]', i.e.,
16 XMM registers.  The padding area is shrunk accordingly.

This issue is minor and has never caused any problems (because of the
padding area), but still fix it.

While there, adjust various whitespace to look much better.