libc: stop trying to create __<syscall> symbols

[brooks]

Summary

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.

[imp]

I have half a memory of this pattern being added to the mix as a compat bridge when moving away from libcR... but it's clear they'd have no effect now