MFamd64: Use initializecpu() to set various model-specific registers on
AP startup and AP resume (it was already used for BSP startup and BSP
resume).

• Split code to do one-time probing of cache properties out of initializecpu() and into initializecpucache(). This is called once on the BSP during boot.
• Move enable_sse() into initializecpu().
• Call initializecpu() for AP startup instead of enable_sse() and manually frobbing MSR_EFER to enable PG_NX.
• Call initializecpu() when an AP resumes. In theory this will now properly re-enable PG_NX in MSR_EFER when resuming a PAE kernel on APs.