cmd/link: use m to store div/mod denominator on arm

<pre>Bring back CL 19810043.
However, also:

1. Disallow use of DIV/MOD in NOSPLIT functions.
That will catch functions that are running without a valid m.

2. Rewrite the functions that are caught (usleep, various 64-bit atomics)
to use magic multiplies instead of constant division.</pre>
