arm64: generate 64-bit syscall.tbl

Change the asm/unistd.h header for arm64 to no longer include
asm-generic/unistd.h itself, but instead generate both the asm/unistd.h
contents and the list of entry points using the syscall.tbl scripts that
we use on most other architectures.

Once his is done for the remaining architectures, the generic unistd.h
header can be removed and the generated tbl file put in its place.

The Makefile changes are more complex than they should be, I need
a little help to improve those. Ideally this should be done in an
architecture-independent way as well.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>