freebsd32: rename freebsd4_freebsd32_sigreturn arg

Rename struct freebsd4_freebsd32_ucontext to struct freebsd4_ucontext32
allowing conversion from the default ABI's struct freebsd4_ucontext
by appending "32". This has no practical effect as this type does not
actually exist.

Give freebsd4_freebsd32_sigreturn an ANSI C prototype.

Reviewed by: kevans