Commit 036fb8770 ("checksum: add VSX fast path for POWER8/POWER9") changed
an #ifdef to #if defined, in order to match a newly introduced #if which
needs to check two different symbols.  This causes clang-tidy to complain
that the directive could be written more concisely.

In this case consistency with the other #if branch seems more important,
and in general insisting on #ifdef over #if seems unhelpfully pedantic.
Suppress that warning globally.

Reported-by: Laurent Vivier <lvivier@redhat.com>
Fixes: 036fb8770cc2 ("checksum: add VSX fast path for POWER8/POWER9")
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Currently we configure clang-tidy with a very long command line spelled out
in the Makefile (mostly a big list of lints to disable).  Move it from here
into a .clang-tidy configuration file, so that the config is accessible if
clang-tidy is invoked in other ways (e.g. via clangd) as well.  As a bonus
this also means that we can move the bulky comments about why we're
suppressing various tests inline with the relevant config lines.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>