amd64 sendsig: fix context corruption

Drop fpstate only after copying out xfpustate from the thread usermode
save area. Otherwise a context switch between get_fpcontext(), which now
returns the pointer directly into user save area, and copyout, would
cause reinit of the save area, loosing user registers.

Reported, reviewed, and tested by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D32159