docs: align with scripts/syscall.tbl migration

Update the documentation to reflect the migration of the following
architectures to the centralized syscall table format:

  arc, arm64, csky, hexagon, loongarch, nios2, openrisc, riscv

As of commit 3db80c999debbad ("riscv: convert to generic syscall table"),
these architectures no longer rely on include/uapi/asm-generic/unistd.h.
Instead, syscall table headers (syscall_table_{32,64}.h) are generated by
scripts/syscalltbl.sh based on entries in scripts/syscall.tbl, with ABIs
specified in arch/*/kernel/Makefile.syscalls.

For the convenience of developers working with older kernel versions, the
original documentation is fully retained, with new sections added to
cover the scripts/syscall.tbl approach.

Verified with `make htmldocs`.

Signed-off-by: Jesung Yang <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>
Message-ID: <[email protected]>

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