Removed unused special fork() implementations.

[brooks]

Summary

The arm version hasn't been used in ages.

The mips version uses a valid, but pointless check of v1 and has been
unhooked from the build since r276630.

[brooks]

The custom assembly for MIPS is designed to support this behavior (from a comment in Ovfork.S):

* v1 == 0 in parent process, v1 == 1 in child process.
* v0 == pid of child in parent, v0 == pid of parent in child.

FreeBSD's kernel actually implements (i.e. v0 is 0 not the ppid in the child)

/*
 * Set up return-value registers as fork() libc stub expects.
 */
td2->td_frame->v0 = 0;
td2->td_frame->v1 = 1;
td2->td_frame->a3 = 0;

[emaste]

Seems fine