nios2: 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.

nios2 has one extra system call that gets added to scripts/syscall.tbl.

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/nios2 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>