[PATCH] x86_64: don't save eflags in x86-64 switch_to()
As discussed, the flags register on x86-64 is saved and restored by the
assembly code which sets up struct pt_regs, so we do not need to save
and restore it in the inline assembler which already informs gcc that
we're clobbering the flags. This patch has been sanity booted and works
okay here.