linux-user: add strace support for openat2

linux-user: add strace support for openat2

This commit adds support for the `openat2()` to `QEMU_STRACE`. It
will use the `openat2.h` header if available to create user
readable flags for the `resolve` argument but does not require
the header otherwise.

It also makes `copy_struct_from_user()` available via `qemu.h`
and `open_how_ver0` via `syscall_defs.h` so that strace.c can use
them.

Signed-off-by: Michael Vogt <mvogt@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <f02d40c7751c03af885ced6dd94e4734d4be4d8f.1727795334.git.mvogt@redhat.com>
[rth: Add braces around the expanded how structure, like strace(3)]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>