{amd64,i386}/SYS.h: add _SYSCALL and _SYSCALL_BODY

Add a _SYSCALL(name) which calls the SYS_name syscall. Use it to add a
_SYSCALL_BODY() macro which invokes the syscall and calls cerror as
required. Use the latter to implement PSEUDO() and RSYSCALL().

Reviewed by: imp, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43059