csky: convert to generic syscall table

The uapi/asm/unistd_32.h and asm/syscall_table_32.h headers can now be
generated from scripts/syscall.tbl, which makes this consistent with
the other architectures that have their own syscall.tbl.

csky has two architecture specific system calls, which I add to
the generic table.  The time32, stat64 and rlimit entries in the
syscall_abis_32 line are for system calls that were part of the generic
ABI when arch/csky got added but are no longer enabled by default for
new architectures.

Both the user visible side of asm/unistd.h and the internal syscall
table in the kernel should have the same effective contents after this.

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