Save and restore floating point registers in get/set_mcontext().

arm64 and riscv were only saving and restoring floating point
registers for sendsig() and sys_sigreturn(), but not for getcontext(),
setcontext(), and swapcontext().

While here, remove an always-false check for uap being NULL from
sys_sigreturn().

Reviewed by: br, mhorne
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23218