libc: stop trying to create __<syscall> symbols

When we switched to an interposing table (commit 8495e8b1e9e1) for
cancelation points we stopped having concrete implementations of
<syscall> and instead use sys_<syscall> and __thr_<syscall>.
These entries of the form:

weak_reference(sys_<syscall>, __<syscall>);

seem to be intended to preserve these symbols, but they have no effect
as __sys_<syscall> isn't defined in the translation units in question.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D51668