Fix C++ build issues w/ LLVM 14 (C++17 support)

[andreittr]

A set of fixes to enable building alongside LLVM 14 libcxx & friends:

• wrong macro set in build options
• missing declaration for prctl

This is part of a patch set consisting of:

• libcxx
• libcxxabi
• lib-compiler-rt
• libunwind
• musl

Tested with GCC 11, 12 & 13, and clang 16.

edit: rebased & removed already upstreamed patches edit2: rebased

[andreittr]

@andreittr the ioctl signature commit is already covered by this, please remove that one.

Thanks for the heads-up, removed and rebased.