MFC r294311:
Clear whole XMM register file instead of only XMM0. Also clear x87
registers. This brings amd64 on par with i386, providing consistent
initial FPU state.

PR: 206370

MFC r294312:
Use ANSI definitions. Wrap long line.

MFC r294313:
Adjust i386 comment to match amd64 one after r294311.

Approved by: re (gjb)