riscv/SYS.h: implement _SYSCALL_BODY() macro

Add _SYSCALL_BODY() macro which invokes the syscall via _SYCALL() and
calls cerror as required. Use to implement PSEUDO() and RSYSCALL().

Reviewed by: imp, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43057