[libc] avoid setting errno in `src/__support`

[michaelrj-google]

Setting errno in support makes controlling whether errno is set much more difficult. Errno should be set as close to the return of a public function as possible.

List of places that need to be changed:

• __support/OSUtil/linux/fcntl.cpp - change to returning ErrorOr since if there's an error it always returns -1.
• __support/OSUtil/linux/vdso.cpp - currently calling public functions in support, which is also a problem. Move getauxval to an internal utility that doesn't set errno and this problem goes away.
• __support/HashTable/randomness.h - same issue as vdso, calling public functions leads to setting errno.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Michael Jones (michaelrj-google)

Setting errno in support makes controlling whether errno is set much more difficult. Errno should be set as close to the return of a public function as possible.

List of places that need to be changed:

• __support/OSUtil/linux/fcntl.cpp - change to returning ErrorOr since if there's an error it always returns -1.
• __support/OSUtil/linux/vdso.cpp - currently calling public functions in support, which is also a problem. Move getauxval to an internal utility that doesn't set errno and this problem goes away.
• __support/HashTable/randomness.h - same issue as vdso, calling public functions leads to setting errno.