asm-generic: add a generic unistd.h

A new architecture should only define a minimal set of system
calls while still providing the full functionality. This version
of unistd.h has gone through intensive review to make sure that
by default it only enables syscalls that do not already have
a more featureful replacement.

It is modeled after the x86-64 version of unistd.h, which unifies
the syscall number definition and the actual system call table
in a single file, in order to keep them synchronized much more
easily.

This first version still keeps legacy system call definitions
around, guarded by various #ifdefs, and with numbers larger
than 1024. The idea behind this is to make it easier for
new architectures to transition from a full list to the reduced
set. In particular, the new microblaze architecture that should
migrate to using the generic ABI headers can at least use an
existing uClibc source tree without major rewrites during the
conversion.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>