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