signal: use BUILD_BUG() instead of _NSIG_WORDS_is_unsupported_size()

Kill _NSIG_WORDS_is_unsupported_size(), use BUILD_BUG() instead.  This
simplifies the code, avoids the nested-externs warnings, and this way we
do not defer the problem to linker.

Also, fix the indentation in _SIG_SET_BINOP() and _SIG_SET_OP().

Note: this patch assumes that the code like "if (0) BUILD_BUG();" is
valid.  If not (say __compiletime_error() is not defined and thus
__compiletime_error_fallback() uses a negative array) we should fix
BUILD_BUG() and/or BUILD_BUG_ON_MSG().  This code should be fine by
definition, this is the documented purpose of BUILD_BUG().

[sfr@canb.auug.org.au: fix powerpc build failures]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>