add configuration option to allow emulating Pentium MSRs but ignore
(rather than fault) RDMSR/WRMSR on undefined registers. Apparently this
allows the 3Dfx GLIDE2.OVL driver to work, since the driver apparently
assumes that Pentium Pro/Pentium II MTRR registers are present despite
the fact that DOSBox *clearly* indicates through CPUID that MTRRs are
not present.