hexagon: use new system call 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.

The time32, stat64, rlimit and renameat entries in the syscall_abis_32
line are for system calls that were part of the generic ABI when
arch/hexagon got added but are no longer enabled by default for new
architectures.

As a side-effect, calling an unimplemented syscall now return -ENOSYS
rather than branching into a NULL pointer.

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