riscv/SYS.h: implement _SYSCALL_BODY() macro

[brooks]

Summary

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

Sponsored by: DARPA

[brooks]

This is part of a set of changes I've posted at https://github.com/brooksdavis/freebsd/tree/SYS.h-unification. I've done basic smoke testing under qemu.