vkernel64: fix compilation after npx mask work
vkernel64: support mxcsr mask on vkernel64 too
kernel: GC never true CPU_DISABLE_SSE checks from x86_64/vkernel64.

It is only an option in i386.

No functional changes.

Reported-by: profmakx
kernel/vkernel: port mxcsr mask probing to i386.
kernel/npx: Enable printing of npx mask on bootverbose on i386