Fix the implementations of PSEUDO_NOERROR and PSEUDO.

The PSEUDO* macros should not declare <syscall>, only _<syscall> and
__sys_<syscall>. This was causing the interposing C wrappers to be
ignored due to link order.

Reviewed by: kib
Obtained from: CheriBSD (4e8e13c90fc6a80e1520de44a6864cfd78b3b56d)
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D4097