These aren't exposed on non-MIPS musl targets, but since they're
part of a kernel API, they're still applicable, so we just don't
test them there but expose them anyways.

Signed-off-by: Tom Kirchner <tjk@amazon.com>
Signed-off-by: Ben Cressey <bcressey@amazon.com>

Also move them to the common Linux module, as they are not platform-specific.
See include/uapi/linux/falloc.h.

It needs to be skipped during testing because there's no way to include
a combination of headers that will expose it without giving a "type has
already been defined" error.

We need to create an initialized sigaction struct for Linux syscalls
thus its fields need to be public.

The field was named _restorer before but system libc/kernel uapi name the field
sa_restorer.

musl has a different aiocb definition; netbsd and OSX have some symbol renaming, and
lwpid_t is signed on freebsd