libc: Use __weak_symbol for poll/ppoll wrappers

[jhb]

Summary

GCC only does not expand the argument to #pragma weak and instead
requires its argument to be a literal string. Use an attribute on the
function definition via __weak_symbol instead.

Fixes: 88276dfbf19e include: ssp: fortify poll/ppoll from <poll.h>

[jhb]

Nevermind, didn't see that you'd already fixed this.