asm-generic: add generic ABI headers

These header files are typically copied from an existing architecture
into any new one, slightly modified and then remain untouched until
the end of time in the name of ABI stability.

To make it easier for future architectures, provide a sane generic
version here. In cases where multiple architectures already use
identical code, I used the most common version. In cases like
stat.h that are more or less broken everywhere, I provide a
version that is meant to be ideal for new architectures.

Signed-off-by: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>