lib/libc/amd64/string: implement strsep() through strcspn()

[fuz]

Summary

The strsep() function is basically strcspn() with extra steps.
On amd64, we now have an optimised implementation of strcspn(),
so instead of implementing the inner loop manually, just call
into the optimised routine.

Sponsored by: The FreeBSD Foundation.

Test Plan

no new test suite failures when substituting this
implementation.