Remove __nonnull() attributes from x86 machine check architecture.

Remove __nonnull() attributes from x86 machine check architecture.

These are of the few cases where we use the GCC non-null attributes in
non-header code. As part of a review [1] of our use of such attributes we
are replacing such uses of the overly aggressive GCC attribute with clang's
_Nonnull attribute.

In this case the attributes serve little purpose as they just don't
enforce run time checks, If anything the attributes would cause NULL pointer
checks to be ignored but there are no such checks so only effect is
cosmetic.

The references appear to be left over from code development and likely
already fulfilled their purpose.

Reference [1]:
https://reviews.freebsd.org/D9004

Reviewed by:	jhb
MFC after:	3 weeks