x86/signal: Move siginfo field tests

Move the tests to the appropriate signal_$(BITS).c file.

Convert them to use static_assert(), removing the need for a dummy
function.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20221219193904.190220-2-brgerst@gmail.com
Cc: Al Viro <viro@zeniv.linux.org.uk>