syscalls: add generic scripts/syscall.tbl

The asm-generic/unistd.h header still follows the old style of defining
system call numbers and the table. Most architectures got the new
syscall.tbl format as part of the y2038 conversion back in 2018, but
the newer architectures that share a single table never did.

I did a semi-automated conversion of the asm-generic/unistd.h contents
into a syscall.tbl format, using the ABI field to take care of all
the relevant differences that are encoded using #ifdef checks in the
existing header.

Conversion of the architectures is done one at a time in order to
be able to review or revert them as needed.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>