added more perfmon MSR defines into cpu.h

git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12507 6c8e4198-a580-4521-a02f-c9ae86df1db9
added comment to RDPMC instr

git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12508 6c8e4198-a580-4521-a02f-c9ae86df1db9