tools/build: tweak unused value workaround

tools/build: tweak unused value workaround

Clang has -Wself-assign enabled by default under -Wall, which always
gets -Werror'ed on this file, causing sync-compare-and-swap to be
disabled by default.

The generally-accepted way to spell "this value is intentionally
unused," is casting it to `void`.  This is accepted by both GCC and
Clang with -Wall enabled: https://godbolt.org/z/qqZ9r3

Signed-off-by: George Burgess IV <gbiv@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Link: http://lkml.kernel.org/r/20200414195638.156123-1-gbiv@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>