gccgo compile fix and support for perfstat_cpu_util

[chbuescher]

when compiling with gccgo - for cpus < power8 i've got an error:
vendor/github.com/power-devops/perfstat/types_fs.go:130:22: error: integer constant overflow
130 | case f.Flags&VFS_DIO != 0:

The other commit is a feature: To get the correct cpu usage of an lpar i had to call perfstat_cpu_util()
I implemented only the TotalStat, not the usage per cpu

[aklyachkin]

thank you very much for the fix and the pull request!

regarding your feature request, I think it repeats the same functionality as in CpuUtilStat(). If we split CpuUtilStat() in 2 functions - one gets old CPU data (what you've done in init()) and the second calculates the difference (your CpuUtilTotalStat()), and then you can use the first function whenever you need it to start counting, would it suit your needs?

Not everyone uses libperfstat to get CPU data from it and that is my problem with init(). Why should we save some data in a global variable for a possible future use?