tools headers: Go back to include asm-generic/unistd.h for arm64

The header unistd.h is included under Arm64's uAPI folder (see
tools/arch/arm64/include/uapi/asm/), but it does not include its
dependent header unistd_64.h.

The intention is for unistd_64.h to be generated dynamically using
scripts/Makefile.asm-headers.

However, this dynamic approach causes problems because the header is not
available early enough, even though it is widely included throughout
tools.

Using the perf build as an example:

1) Feature detection: Perf first runs feature tests.

The BPF feature program test-bpf.c includes unistd.h. Since
unistd_64.h has not been generated yet, the program fails to build,
and the BPF feature ends up being disabled.

2) libperf build:

The libperf Makefile later generates unistd_64.h on the fly, so
libperf itself builds successfully.

3) Final perf build:

Although the perf binary can build successfully using the generated
header, we never get a chance to build BPF skeleton programs,
because BPF support was already disabled earlier.

Restore to include asm-generic/unistd.h for fixing the issue. This
aligns with most architectures (x86 is a special case that keeps
unistd_32.h/unistd_64.h for its particular syscall numbers) and ensures
the header is available from the start.

Fixes: 22f72088ffe69a37 ("tools headers: Update the syscall table with the kernel sources")
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>