KVM: MMU: mark role_regs and role accessors as maybe unused

It is reasonable for these functions to be used only in some configurations,
for example only if the host is 64-bits (and therefore supports 64-bit
guests).  It is also reasonable to keep the role_regs and role accessors
in sync even though some of the accessors may be used only for one of the
two sets (as is the case currently for CR4.LA57)..

Because clang reports warnings for unused inlines declared in a .c file,
mark both sets of accessors as __maybe_unused.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>