[kernel] Set framebuffer to write-combining

Several changes were needed to make this work:

- Update the page_table::flags to understand memory caching types
- Set up the PAT MSR to add the WC option
- Make page-offset area mapped as WT
- Add all the MTRR and PAT MSRs, and log the MTRRs for verification
- Add a vm_area flag for write_combining

src/kernel/msr.cpp

@@ -1,5 +1,11 @@
 #include "msr.h"
 
+msr
+find_mtrr(msr type, unsigned index)
+{
+	return static_cast<msr>(static_cast<uint32_t>(type) + (2 * index));
+}
+
 uint64_t
 rdmsr(msr addr)
 {