stdlib.h: Partially revert c27a89971805

quick_exit() can call other functions, and we don't guarantee it calls
std::terminate should those other functions throw exceptions. And to
make it do so has ABI complications for libc. Until that's sorted out,
revert this noexcept (but leave a comment behind so people will find
this commit message)

Requested by: kib

Sponsored by: Netflix