Move CPU softfloat unions to cpu-float.h

The types are no longer used in bswap.h since commit
f930224f ("bswap.h: Remove unused float-access functions"), there
isn't much sense in keeping it there and having a dependency on fpu/.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220323155743.1585078-29-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>