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

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
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42346

(cherry picked from commit fd2ecd91aeeeab579c769c9a39f90b4bd4a493a9)