locking/atomics: build atomic headers as required

locking/atomics: build atomic headers as required

Andrew and Ingo report that the check-atomics.sh script is simply too slow
to run for every kernel build, and it's impractical to make it faster
without rewriting it in something other than shell.

Rather than committing the generated headers, let's regenerate these
as-required for a pristine tree.

That ensures they're always up-to-date, allows them to be built in
parallel, and avoid redundant rebuilds, which is a 2-8s saving per
incremental build.  Since the results are not committed, it's very obvious
that they should not be modified directly.  If we need to generate more
headers in future, it's easy to extend Makefile.genheader to permit this.

I've verified that this works in the cases we previously had issues with
(out-of-tree builds and where scripts have no execute permissions), and
have tested these cases for both x86_64 and arm64.

The diffstat looks nice, at least...

Link: http://lkml.kernel.org/r/20181123153321.8561-1-mark.rutland@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>